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In the top of a double boiler over simmering water, melt the chocolate, stirring occasionally.
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Remove from the water and cool the chocolate until tepid.
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In a bowl, whisk the flour, baking powder, and salt together; set aside.
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In a glass measuring cup, stir the espresso into the milk until dissolved; set aside.
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In a bowl, beat the sugar and shortening together using a hand-held electric mixer, until mixture is well combined.
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Beat in the melted chocolate.
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Beat in the eggs, one at a time, and the vanilla.
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Decrease mixer speed to low--one-third at a time, alternately beat in the flour and milk mixtures.
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Stir in the walnuts.
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Cover with saran wrap and refrigerate until chilled and firm (about 2 hours).
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Roll dough by tablespoonfuls into 1-inch balls.
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Roll the balls in the powdered sugar to coat, then place 2-inches apart on nonstick baking sheets.
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Bake in a 350° oven for about 15 minutes or until cookies are set and edges are crisp.
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Cool on baking sheets for 5 minutes; transfer to wire rack to cool completely.