1. Preheat the oven to 180C/160C fan/gas 4. Butter and flour the sides of two 18cm sandwich tins and line the bases with a disc of baking parchment.
2. Cream the butter until soft in a large bowl or in an electric food mixer. Add the sugar and beat until the mixture is light and fluffy.
3. Whisk the eggs together in a small bowl for a few seconds until just mixed, then gradually add them to the butter mixture, beating all the time. Sift in the flour and baking powder, then add the milk, lemon zest and juice and fold in gently to incorporate.
4. Divide the mixture between the two tins and make a slight hollow in the centre of each cake so that when it rises it doesnt peak too much in the centre, making it difficult to sandwich together with the other half.
5. Place in the centre of the oven and bake for 18-25 minutes, or until golden on top and springy to the touch.
6. Remove from the oven and allow to cool in the tins for 10 minutes, then loosen around the edges of each cake using a small, sharp knife and carefully remove from the tins before leaving on a wire rack to cool down completely.
7. For the Buttercream, whisk softened butter in mixing bowl until smooth and creamy. Add icing sugar, lemon curd and lemon zest. Cover with a tea towel if possible to save icing sugar going everywhere. Whisk until everything is incorporated.
8. Once the cake is cool spread the buttercream between the layers.
Tip: If you have left over buttercream it will freeze!