Directions

1. Line the cake tins with baking parchment and butter the bases and sides well. Heat oven to 190C/170C fan/gas 5. Combine all the cake ingredients in a freestanding mixer until smooth and pale, starting at a low speed as the bowl will be very full; or use an electric handwhisk and a large bowl, if you prefer.
2. Set the 25cm cake tin on scales and pour in 1.5kg batter. Do the same with the 20cm tin and pour in the rest (it should be about 1kg).
3. Bake the cakes in the oven for 1 hr or until risen, golden and a skewer inserted into the middle of the cake comes out clean (see tip). Cool the cakes in their tins for 10 mins, then turn out onto wire racks to cool completely. Can be made ahead and frozen.
4. To make the buttercream, cut the butter into pieces, add about 1/3 of the icing sugar and beat well with an electric handwhisk. Once fully combined, add the next 1/3 and beat again. Repeat once more with the remaining sugar, the milk and lemon zest, and keep beating until pale. For the filling, stir the lemon juice and curd together in another bowl and set aside until needed.
5. To assemble the cake, trim the tops and cut both cooled sponges in half horizontally using a serrated knife. Spread just over half the filling on top of one of the larger sponges and the rest over a smaller one. Leave for 5 mins. Spread 250g buttercream over the filling on the larger sponge and sandwich the other larger sponge on top. Do the same with the smaller sponges, using 150g buttercream.
6. Stick your cakes onto cake boards using a small blob of buttercream under each. Spread some of the remaining buttercream all over the tops and sides of both cakes in a really thin layer - this will bind with loose crumbs to create an undercoat for your chosen decoration. Put both cakes in the fridge for 1 hr to firm up.
7. To stack the cakes, push a straw down into the centre of the larger cake. Snip the top off with scissors so that it's flush with the buttercream layer. Add five more straws around the central one, equally spaced apart. Carefully place the smaller cake on top, then spread the rest of the buttercream all over the surface to cover it or fill in any gaps.