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In a small bowl, stir together the flour, cocoa powder, coffee crystals, and salt.
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Set the mixture aside.
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In a large mixing bowl, beat the butter with an electric mixer on medium to high speed for 30 seconds.
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Add the 1/2 cup powdered sugar and vanilla; beat the mixture well.
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Add flour mixture, beating on low to medium speed until combined.
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Stir in walnuts with a wooden spoon.
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Shape dough into 1-inch balls.
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Place balls about 1-1/2 inches apart on ungreased cookie sheets.
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Bake in a 325F oven for about 15 minutes or until the cookies are set.
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Cool on a cookie sheet for 1 minute.
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Transfer the cookies to wire racks.
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While the cookies are still warm, roll in or sprinkle with some additional powdered sugar.
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Cool, then roll again in powdered sugar before serving.
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Store in a tightly covered container.