Preheat oven to 160C. Line a 6 cup muffin tin with patty cases
Use an electric mixer, cream the butter and sugar until light and creamy. Add the egg and beat until well combined. Add the sour cream and the flours and stir to combine. Spoon the mixture evenly into the cases.
Place in a tray and bake for 20 minutes or until cooked through. Remove from oven and transfer cakes to a wire rack to cool completely.
Make icing – place icing sugar in a bowl, add1 a tablespoon of soft butter, mix the butter into the icing sugar until combined, add the lemon juice and stir until firm paste forms. Ice the top of the cakes.