In a bowl combine flour, corn starch and baking powder and set aside.
In a large bowl whisk together melted butter and sugar, until combined. Add egg, lemon extract and zests and whisk again until smooth. Gradually add the dry ingredients, stirring gently with a wooden spoon, until absorbed. A soft dough should form.
Leave the dough covered with cling film for 30-60 minutes in the fridge.
Preheat oven to 180 degrees C (350 degrees F). Line a large baking tin with a sheet of parchment paper.
Roll and cut out cookies. Arrange them on the baking sheet. Bake for about 10 minutes on the top rack of the oven, or until the edges start browning. Optionally you can decorate the cookies with sprinkles before baking them or with fondant/ royal icing after baking.