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Preheat oven to 350° and grease 3 or 4 baking sheets.
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In a bowl, beat butter and sugar until light and fluffy.
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In separate bowl, beat egg with coffee granules until dissolved. Gradually add to creamed mixture, beating well.
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Sift the flour, cornstarch and baking powder over mixture.
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Stir in to make a soft dough and knead lightly.
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On a floured surface, roll dough to a 1/8-inch thickness.
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With a 2-inch fluted cutter, cut out cookies and place on prepared baking sheets.
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Bake 10-15 minutes, until firm and lightly browned.
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Transfer cookies to wire racks to cool.
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Put the cooled, melted chocolate into a pastry bag and snip off end.
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Pipe parallel zigzag lines on half the cookies.
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For cookie filling: In bowl, beat butter and powdered sugar until smooth and creamy.
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In another bowl, mix egg yolk with coffee granules; add to creamed mixture and beat until smooth.
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Spread the filling on plain cookies and then sandwich together.
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Let cool in cool place until filling is firm.