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 5cm apart on an unprepared baking tray. Make an indentation in the center of each ball with your finger. Bake for 10 to 15 minutes in the preheated oven.
While the biscuits are baking melt the chocolate chips in a small saucepan over low heat or in the microwave. Stir chips until smooth. When biscuits come out of the oven spoon 1/2 teaspoon of the melted chocolate into the indentation. When biscuits are completely cool dust lightly with icing sugar.