Line two 20cm/8 inch springform pans with baking paper on the base and sides. Preheat oven to 160C/320F.
Beat the butter and sugar until pale and fluffy. Add the eggs in one at a time and then fold the flour and baking powder in followed by the milk and zest. Spoon into prepared pans and bake for 40 minutes until a skewer inserted in the centre comes out clean. Cool completely.
Whip the cream with the extra sugar until it reaches peaks that can hold.
Trim the tops of the cakes and then using dental floss or a knife, slice through each cake horizontally so that you have four layers.
Spread the bottom layer with the whipped cream, then fill the middle with the 1/2 cup of lemon curd and then the cream for the top layer. Mix the remaining 1/4 cup lemon curd with the rest of the whipped cream and swirl. Dollop on top of the cake and form peaks with a spatula. Sprinkle with the lemon sherbert just before serving.