Knead the powdered sugar, butter, starch and flour into a dough. Shape the dough into small balls and use a wooden spoon handle to make a recess in each of the balls.
Bake at 160 degrees (convection) for about 10 minutes, then let the cookies cool down.
Melt the chocolate in a bain-marie, stir in the peppermint oil and pour some of it into the wells of the biscuits. Let cool down.