Preheat the oven to 180ºc/160º fan/Gas 4/350ºF Grease and line two 8" (20cm) round loose-bottomed cake tins.
Put the soft butter (225g) into a large mixing bowl, add the caster sugar (225g) and use a handheld electric mixer to cream together until light and fluffy.
Add the eggs to the mixing bowl, one at a time, beating well after each addition.
Place a sieve over the mixing bowl and sift in the flour (225g) and baking powder (2tsps).
Use a large metal spoon to gently fold the ingredients together. Take care not to overmix.
Add the grated lemon zest and lemon juice to the mixing bowl and gently fold through the cake batter.
Divide the mixture evenly between the two tins. Smooth the top of the mixture with the back of a spoon and gently tap the cake tin on the work surface to release any air bubbles.
Bake in the oven for 25 minutes. When baked, the cakes will shrink away from the sides of the tin slightly and will feel springy to the touch. Remove from the oven and leave to cool in the cake tins before turning out.
When the cakes are cool, release them from the cake tins. Fill and decorate with lemon buttercream (recipe link in post and notes).