Using a stand mixer or hand beaters, cream butter and sugar until light and pale. Add egg and yolk and mix well to completely combine.
Add all dry ingredients and mix to make a stiff dough. Roll into a ball, wrap in plastic wrap and refrigerate for 30 minutes.
Preheat oven to 180C.
Taking walnut-sized pieces of dough, roll into balls and place on lined baking tray. Press dough balls down firmly with a fork, then sprinkle with demerara sugar.
Alternatively, you can roll the dough out thinly and cut with a cookie cutter, then sprinkle with the sugar.
Bake for 15 minutes, or until golden.