In a large bowl (or a stand mixer) beat butter and sugar until light and fluffy
Add vanilla, lemon peel and egg yolk. Mix well
Lightly spoon flour into measuring cup and level off.
Mix in flour and ground hazelnuts
Shape dough into 1 1/2 inch balls. Place 2 inches apart on an ngreased cookie sheet
Make an indent in each ball with your thumb, creating a pool in the centre
Bake your cookies at 325 degrees F for 10 to 15 minutes, until light golden brown around the edges.
Melt chocolate chips in a microwave or on the stove top in a double boiler.
Spoon about 1/2 teaspoon melted chocolate into the center of each cookie sheet.
If you have leftover hazelnuts, you can roast them in the oven for 8 minutes (until skin cracks) and chop them up to decorate the chocolate with.