Preheat the oven to 180 C / Gas 4. Lightly grease and line two 20cm sandwich tins.
For the cake:
Sift the flour and caster sugar into a large bowl. Add eggs, butter or margarine, lemon zest, vanilla extract and baking powder and mix with a wooden spoon until smooth and creamy. Divide the mixture between the two prepared sandwich tins.
Bake for 40 to 45 minutes until risen and golden and a skewer inserted into the centre comes out clean. Remove the cakes from the tins, remove lining and leave on a wire rack to cool. I put one of the cakes back into its tin and use the edge to rest a knife on to trim the cake to make the base flat.
For the filling:
In large bowl, beat together the icing sugar, unsalted butter, lemon juice and lemon zest together until smooth. Beat in milk, then increase speed and continue to beat until light and fluffy. Spoon into a piping bag.
Lay the base cake onto a serving plate then spread the lemon curd on top. Pipe the icing on top to prevent the curd from being pushed out, then smooth this lightly too. Decorate with thinly sliced strawberries, put the top cake on then dust with icing sugar.
I was given the tip that the best sponge is from breaking 4 eggs into a bowl, weighing them and then use the same volume of sugar, flour and butter, perfect every time!