Cream butter, Icing Sugar and egg yolks until light and creamy. Add flour and polenta and mix to a soft dough. Rest for 30 minutes in fridge. Roll out between baking paper and line a 25cm to 28cm flan pan. Place in freezer for ½ hour. Cover pastry with baking paper and line with rice or beans to ensure pastry retains it shape. Bake at 200 ºC conventional or 180ºC fan forced for 15 minutes. Remove rice / beans and cook for a further 5 minutes.