In a bowl cream the butter and sugar together on medium high speed. Beat in the egg whites, one at a time. Beat in the vanilla.
Lower the speed and add the flour mix until just combined. Do not overmix. Cover and chill for at least 1 hour.
Preheat oven to 170 degrees C.
Cut a small hole (about 90mm diameter) in a thin piece of cardboard or plastic to serve as a template in forming the biscuits. Line a baking tray with baking paper or a silicon mat and place the stencil on it. Using a spatula, place a small amount of the batter in the centre of the hole of the stencil and spread it out evenly. Carefully lift the stencil off. Repeat for more biscuits.
Bake for 8 to 10 minutes or until lightly brown around the edges. Remove from the oven and while still hot, remove them from the pan and place them either over a rolling pin, over a glass or in the hollows of an empty egg carton. Allow to sit a few minutes to harden and cool. Store in an airtight container.