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Preheat oven to 350 degrees. Line 2 baking sheets with silicon baking mats.
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Cream together the butter and sugar on medium speed with a hand held mixer, or in the bowl of an electric mixer until light and fluffy. Add vanilla and lemon zest.
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In a medium bowl, combine flour and salt.
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Add dry ingredients slowly into butter mixture and combine. Do not overmix.
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Dump batter onto flour counter and flatten into a disk. Wrap with plastic wrap and refrigerate for 60 minutes.
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Beat the egg in a medium bowl and set aside. Place the chopped walnuts in a separate medium bowl. Roll the batter into small balls and dip each one first into the beaten egg, letting the excess drip off, and then rolling each cookie in the chopped walnuts.
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Place the cookies on the cookie sheet about 2 inches apart. I typically get about 14-16 cookies per large sheet. Use the tip of your finger to make an impression/indentation in the center of each cookie and fill with 1/4 tsp. of jam.
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Bake for 15-16. Watch the cookies to make sure they don't burn. If you like them a little crisp, bake for up to 17 minutes, but watch them carefully.
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Cool on wire rack...dust with Confectioner's sugar if desired.