Preheat the oven to 180C / 350F. In a medium sized bowl or the bowl of your stand mixer add the room temperature butter and icing sugar / powdered sugar.
Beat on medium speed with a hand mixer for 2-3 minutes or until light and fluffy. Crack in the egg and beat again for a further minute until fully incorporated.
Press the base into a lined 20cm or 8″ square baking pan. Bake the base for 20-25 minutes or until the edges start to brown. Leave to cool in the baking pan.