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Preheat oven to 325 degrees F (165 degrees C).
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In a large bowl, cream butter and sugar.
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Gradually beat in the egg yolk, vanilla, and lemon zest.
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Stir in the flour and ground hazelnuts with a large spoon.
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Shape dough into walnut sized balls and place them 2 inches apart on an ungreased cookie sheet.
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Make an indention in the center of each ball with your finger.
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Bake for 10 to 15 minutes in preheated oven.
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Melt the chocolate chips in a small saucean or microwave.
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Stir chips until smooth.
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When cookies come out of the oven, spoon 1/2 teaspoon of the melted chocolate into the indentation.
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When cookies are completely cool, dust lightly with powdered sugar.