Grease a mini-muffin tin, set aside. Preheat the oven to 176�C.
In a large bowl, cream together the butter and sugar.
Add the egg yolk and vanilla and mix until smooth and fluffy.
Add the flour, baking soda & powder to a small bowl, stir together and then add to the butter mixture.
Mix until well combined (dough may seem crumbly)
Use a small cookie scoop to portion 1 1/2 inch balls of dough, and use your hands to roll them into a smooth ball. (I found the dough seemed dry until I rolled each scoop by hand.)
Portion a dough ball into each of the wells of your pan, and press down slightly.
Bake for 8-10 minutes or until the edges are golden. Allow to cool before removing from the pan.