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.
Melt the chocolate on double boiler together with 2 tbs water.
Gently dip the cookies into the melted chocolate, so that it covers just half of each cookie and only on the top side. Generously sprinkle with the sugar sprinkles on top. Arrange the cookies in a plate to dry out.
Store the cookies in an airtight container for at least 1 week.