Heat oven to 180C/160C fan/ gas 4. Line a 20 x 20cm square baking tin with baking parchment.
Using an electric whisk, beat the butter and sugar together until pale, light and fluffy. Add the eggs and mix again. Add the flour, baking powder, lemon zest, lemon curd and milk, and mix with a wooden spoon until all the ingredients are thoroughly combined. Pour the mixture into the prepared tin and bake for 25-30 mins or until a skewer comes out clean.
Mix the sugar and lemon juice together and pour over the hot cake. Leave to cool in the tin. You can eat the cake as it is, or for a fancy finish, try making this feather icing.
Mix the icing sugar with just enough water to give a runny, but not watery, icing. Put a small amount of icing in a separate bowl. Add a few drops of the food colouring to the icing until pale yellow. Spoon into a disposable icing bag.
Remove the cake from the tin and peel off the baking parchment. Sit the cake on a wire rack over a baking tray. Spread the white icing over the top. Pipe thin lines of the coloured icing across the width of the cake. Use a cocktail stick to drag through the lines in opposite directions to create a feathered effect. Leave to set before cutting into slices.