Lightly grease two 20cm loose bottomed sandwich tins and line the bases with a circle of non-stick baking parchment. Pre-heat the oven to 180C/160F/Gas 4.
Cream the butter and sugar together in a bowl. Beat in the eggs and add the grated lemon zest and mix well. Pour the mixture in a little at a time and mix well. Sift in the flour and baking powder and combine together.
Divide the mixture evenly between the two tins and level with a spatula or the back of a spoon. Bake in the oven for about 25 minutes or until well-risen and golden.
Leave the cakes to cool in the tins for a few moments then run a blunt knife around the edge of the tins to free the sides of the cakes. Turn the cakes out, peel off the paper and leave to cool completely on a wire rack.
As the cakes cool, make the butter-cream by creaming together the butter and icing sugar. Add the milk and lemon juice and zest and beat until smooth.
Choose the cake with the best top, then put the other cake top downwards on to a serving plate. Spread with the lemon curd and then the butter-cream. Put the other cake on top and dust with icing sugar. Put the kettle on, cut a big slice and enjoy!