In a bowl combine flour, corn starch, baking powder and salt and stir to blend.
Break the chocolate into small pieces using a sharp knife.
Melt butter on double boiler (in a heatproof bowl placed over a pan of gently simmering water) and then transfer it to a large mixing bowl. Add the two kinds of sugar and stir with a whisk for 1-2 minutes. Add orange zest and juice, egg and yolk and stir again until the mixture becomes homogeneous.
Gradually add the dry ingredients, mixing gently with a wooden spoon, until a soft dough is formed.
Refrigerate the dough for at least 30 minutes.
Preheat oven to 180 degrees C.
Make small balls out of the dough and place them on a baking tin, lined with parchment paper, not too close one next to another. Bake on the top rack of the oven for 10-12 minutes or until the cookies start browning around the edges. Let them cool until room temperature before decorating them.