Place 200g each softened butter, caster sugar and self raising flour, with 11/2 tsp baking powder and 4 eggs in a food processor and blend until smooth.
Finely grate the rind from 1 lemon and mix in.
Spoon into a lined 450g loaf tin and bake at 180C for 45 minutes until golden and a skewer inserted comes out clean.
Combine the juice of the lemon with 2 tbsp demerara sugar and spoon over the top of the cake whilst hot.
Leave in the tin to cool.
Split the cake lengthways and spread with 3 tbsp lemon curd.