Lemon Ombre Cake (or Happy Birthday to Me!)
There are a few flavours and flavour combinations which always make me happy: a good sharp hit of lemon would probably be in my top 3.  It was my birthday last week and, given that the long Easter weekend was coming right up, I deferred celebrations a little.  It's probably a bit odd to make your own birthday cake, but I consider it a present to myself!  I opted to max out the lemon: light as air lemon sponge layers, silky smooth but cheek-suckingly-sharp lemon curd filling, all swathed in tangy swirls of fluffy lemon frosting.  But the lemon isn't agressive at any point, it's all mellowed and balanced by the sugar you always get (and which can be cloying with other flavour combos) in both the cake and the frosting.  Oh, and all with the fashionable ombre effect (it's de rigeur daaahling!!).  There are a couple of things I would do differently next time I make this cake: firstly I would keep the cake refrigerated right up until serving just so that the cake crumb was firmer; and I would frost the cake on its serving plate rather than try to transfer it once completed...yikes!  If you have sandwich tins your life will be easier as you can bake 2 layers at a time; if you don't it's still not difficult, it just takes a bit longer to get the cake stage done.

For the Cake:
This is a 4 egg mix (from the cake mix matrix which can be found here.  PLUS:
8 drops of yellow food colouring gel (I used Americolor Egg Yellow), 
3 tblspns milk (making 7 tblspns or 105mls total)
Zest of a large lemon.

1. Preheat the oven to 160 C / 325F.  Weigh your mixing bowl and note down the number.  Line two 20cm / 8" sandwich tins, or a single cake tin if that's what you have to hand.  Make the cake batter as per the instructions in the matrix, adding the extra milk and lemon zest, then beat on high for 3 minutes until very pale, very light and very soft.
2. Scrape off the beaters into the bowl and weigh it again, note down the number.  Now, subtract the weight of the mixing bowl and divide the answer by 5 - that's how much mixture is going into each layer of the cake (for me it was 175g / 6oz.  Place the cake tin on the scales, zero the scales, then pour in the required weight of cake batter.  Tilt the cake tin gently until the batter is evenly spread (if your mixture is a bit too firm to tilt into place just nudge it gently with a small spatula).  
3. Add 2 drops of yellow gel to the remaining batter in the bowl and mix VERY well. Add 175g / 6oz of batter to the second tin, level the mixture.  Now bake both of these for approximately 9-10 minutes or until they feel springy to the touch.  If you're baking one layer at time make sure you bake it in the centre of the oven.  Meanwhile add a further 2 drops of food colouring gel to the batter in the bowl and beat well.
4. When cooked, cool the cakes in the tin for about 5 mins then carefully de-tin onto a cooling rack.  Run some cold water into the tin (to cool it down), re-line and weigh in another 175g/6oz of batter.  Repeat the process until you have 5 cooked layers of sponge cake, adding 2 drops of colouring to the batter for each layer.  Make sure you remember the order in which you placed the cooked layers on the cooling racks...so that you stack them in the right order when you assemble the cake!

For the Lemon Curd / Lemon Butter Filling:
2 large or 3 medium lemons
150g / 6oz sugar
100g / 4oz unsalted butter cut up
4 large eggs (don't use very fresh eggs, the egg white is too thick)

1. Place the zest and juice of the lemons in a heatproof bowl.  Add the sugar and the butter.  Break the eggs into jug or bowl, beat them well, then strain them through a sieve onto the other ingredients.  I know this sounds fussy, but you really don't want lumps of cooked egg white making your lemon curd grainy.
2. Place the bowl over a pan of simmering water and stir until everything is melted and/or dissolved - about 5 mins.  At this stage you'll probably wonder how such an unpromising mess will ever turn into something wonderful...trust me...it does.  Just leave the bowl over the simmering water, stir it every couple of minutes while you get on with something else, until you have a thick custard consistency.
3. Remove the bowl from the heat, allow it to cool a little then cover completely with plastic wrap (it's best if the wrap touches the surface of the curd), and place in the fridge until you assemble the cake.

For the Frosting:
200g / 2 sticks unsalted butter, softened
500g / 4 cups icing sugar, sieved
juice of 1 large lemon (the one you zested for the cake)

1. Place all the ingredients in the bowl of a free standing mixer and (using the paddle attachment) beat on slow until everything is combined.  Increase the speed to high and beat for 3-4 mins until very white, light and fluffy.  Scrape down the sides and beat for a further 30 seconds.

To Assemble the Masterpiece:

1. If your cakes have fairly flat tops (which they should) you can go straight on to the layering.  If they have domed tops, carefully slice off the domed part so you have a flat surface.  Place the darkest yellow layer upside down on the serving plate.  Place about 2 tblspns of the lemon curd on top and spread almost to the edges.  If you need a little more lemon curd that's fine, you'll have plenty.
2. Take your next darkest yellow layer and carefully place that on top.  Keep layering up the cakes with lemon curd in between until you have placed your final palest layer on top.  You can of course do this in reverse order so the lightest layer is at the bottom, it just seems a bit counter-intuitive!
3. Now, take a bamboo skewer (or similar implement) and push it, from the top, right down through the entire cake stack.  This will give you some stability while you add the crumb coat of frosting.  Take a small amount of icing and spread it very thinly all over the top and sides of the cake.  Add more icing where you need to so that you have a completely smooth surface over the top and sides.  Remove the skewer and place the cake in the fridge for 20-30 mins.
4. Now for the fun part!  I can't be precise here because so much depends on your icing style.  It really doesn't matter about precision though as you will have plenty of frosting of the right colour when you need it.  So, place a good couple of scoops of white icing in a disposable pastry/icing bag fitted with a coupler and small open star tip.  Starting at the top edge, pipe 6's or 9's (that's left or right spirals) around the side of the cake.  They should be large enough to cover about 1/4 the height of the cake.  Next, pipe the same spirals on top of the cake around the outside edge...see the photo above if my description is inept!  If you run out of frosting just scoop some more into your piping bag from the bowl.  Any frosting left in the bag can be squeezed back into the bowl.
5. Add 1 drop of yellow food gel colouring to the frosting and beat well, it must be completely mixed in with no streaks.  Remove the coupler and tip from the previous icing/pastry bag and place in a new bag.  Scoop in some of the pale yellow frosting and pipe another layer of spirals around the side of the cake just below the white ones, repeat on the top of the cake.  Add another drop of food colouring to the frosting in the bowl, beat really well, then complete your third row...add a drop of colour, beat, complete the fourth and final row.
6. Place the cake in the fridge for 30 mins or so, just to firm up the frosting.  And use a good sharp knife when you cut the cake...I didn't, and it fuzzied up the layers a little.