In the bowl of the robot, add butter and sugar until it make a paste.
Add the yolks one at the time.
In a bowl, mix together, flour, corn starch and baking powder.
Mix the flour mixture into the butter mixture.
Make a ball and refrigerate for half an hour.
Roll into half a cm thick and cut with a cookie cutter. Half full and half with a hole in it if you choose to.
Refregirate the cookies for 1h before baking them.
Bake for 10-12 minutes at 350F (180C).
Allow the cookies to cool down.
Sprinkle icing sugar on the cookies you will use for the top (if you want to).
Add a spoon of jam or chocolate spread on the bottom cookies and put the top on.
