In a large bowl, cream butter and sugar. Gradually beat in the egg yolk, vanilla, and lemon zest. Stir in the flour and ground hazelnuts with a large spoon.
Shape dough into walnut sized balls and place them 2 inches apart on an unprepared cookie sheet. Make an indentation in the center of each ball with your finger. Bake for 10 to 15 minutes in the preheated oven.
While the cookies are baking, melt the chocolate chips in a small saucepan over low heat, or in the microwave. Stir chips until smooth. When cookies come out of the oven, spoon 1/2 teaspoon of the melted chocolate into the indentation. When cookies are completely cool, dust lightly with confectioners' sugar.
