Mix Butter and sugar in TM bowl. Speed 4 for 30 seconds.Add Rice Flour and Plain Flour and mix 20 seconds, speed 3.Knead "Dough mode" 2 minutes in Inverval speed with lid in closed position "Closed lid" to form soft dough.Roll out onto floured surface to desired thickness and cut into shapes.Bake in 180 degree preheated oven until golden brown and sprinkle with extra caster sugar while still hot.Cool on tray for 5 mins and then move to a wire rack to completely cool.