Pre-heat the oven to 180°c
Line a 12 x 9 inch baking pan with baking paper
Place the butter, castor sugar, flour, baking powder, eggs & 3 tbsp of lemon juice in a bowl and blend until smooth.
Spoon the cake batter into the lined baking tin
Pop in the oven for 30 minutes. Test with a skewer, if it comes out clean, its ready.
Allow the cake to cool.
Now make the icing. Mix the icing sugar with a few drops of icing, stiring through until you have a thick, gloopy and smooth consistency
Spoon onto the top of the cake, smooth out, pop the cake in the fridge for 10 minute to help it set.
Cut into squares and serve