1Preheat the oven to 190C/170C Fan/Gas 5.
2Grease a 30cm x 23cm rectangular baking tin. Line with a large piece of baking paper, creating two upright folds across the width to divide the tin into 3 even sections.
3Put the caster sugar and eggs into a glass bowl, then place over a pan of simmering water – don’t let the base of the bowl touch the water. Whisk gently with an electric hand mixer for 2 mins, or until the mixture is foamy and the sugar has dissolved fully. To test, rub a little between your fingers – it should feel smooth, not gritty.
4Carefully remove the bowl from the pan, then stand it on a tea towel on a work surface. Set your mixer to medium-high. Whisk for 5 mins or until the mixture is thick and mousselike and leaves a ribbon trail when the whisk is lifted. Make sure there is plenty of air in it.
5Melt 60g of the butter in a small bowl and allow to cool but not set.
6Meanwhile, divide the sifted flour between 3 separate bowls. Working one at a time, gently fold ⅓ of the egg mixture into each.
7Add 1½tbsp of the melted butter to each of the 3 bowls in turn, mixing gently until combined.
8Add the vanilla extract to one bowl of mixture, the strawberry extract and pink food colouring to another, and the lemon extract and yellow food colouring to the remaining bowl, mixing until well combined.
9Tip each bowl of cake mixture into a separate section of the divided cake tin. Bake for 12-15 mins, until the top springs back when lightly pressed. Leave to cool in the tin for 10 mins, then carefully turn out onto a wire rack and allow to cool fully.
10Meanwhile, make the buttercream. Beat together the icing sugar and remaining butter until light and fluffy. If it’s too stiff, add a little milk until it’s soft and easy to spread.
11Spread the cooled vanilla sponge with ½ the buttercream, then layer the pink strawberry sponge on top. Spread that with the remaining buttercream and top with the yellow lemon sponge. Trim the edges of the cake stack to neaten.
12For the icing, put the chopped fondant into a nonmetallic bowl with 1tbsp water, then microwave for 30 secs at a time, stirring in between, until smooth and melted.
13In a small bowl, mix ¼ of the icing with 1 drop at a time of the pink food colouring, until you have your preferred shade. Leave the remaining ¾ white.
14Spread the white icing over the top of the top sponge layer – take it up to the edges of the cake, but don’t let it drip down the sides.
15Drizzle fine lines of pink icing across the width of the cake, with approximately 1cm between each line.
16To create the feathered effect, very gently drag the point of a cocktail stick along the length of the cake, through the pink lines, in opposite directions. Let the icing set for at least 10 mins, then cut into 6 slices to serve.