Make the sponge batter
Preheat the oven to gas 3, 160°C, fan 140°C. Grease and line 2 x 20cm round baking tins with nonstick baking paper. Beat together 330g softened salted butter and 330g caster sugar with an electric whisk until light and fluffy (about 5 mins). Add 5 large eggs (at room temperature), beating in one at a time until fully incorporated.
In a separate bowl, mix together 330g plain flour, 4 tsp baking powder, the zest of 1 lemon and a pinch of salt. Gradually fold the dry ingredients into the butter and egg mixture, followed by 4 tbsp milk to loosen the batter slightly (it should just drop off the spoon when tapped against the edge of the bowl).