Preheat the oven to 180C.
Choose one large or two smallish silicon cake moulds (butterfly, heart, flower etc) or one cake tin (grease and line it).
Melt the butter and let it cool.
Beat the sugar and eggs in a bowl with an electric mixer for 5 minutes. Add the lemon zest & juice.
Sieve the flour and baking powder into the egg mixture and combine gently into a smooth batter.
Gently fold in the melted butter and milk.
Fill the silicone shape(s) with the batter and put them on a flat baking tray.
Bake for 15-20 minutes in the middle of the oven until the cake is golden and springy. (Test it with a skewer if cooked through.)
Cool in the mould/tin for 5 minutes then turn the cake out onto a wire rack. Let it cool completely.
Glaze and decorate the cake to your liking.