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For the cookies: Mix together the flour, baking powder, and salt.
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In a small pan, heat chocolate over low heat to melt, stirring constantly, let cool slightly.
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Cream butter using an electric mixer, then beat in sugar until fluffy.
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Beat in the melted chocolate, the vanilla, and the egg until well combined.
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Using a wooden spoon, stir in the flour mixture until combined.
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Divide dough into 4 balls, wrap in plastic wrap andpop into the freezer for 30 minutes (or into the fridge for 1 hour).
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Preheat the oven to 350F and have ready parchment paper-lined baking sheets.
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Shape one ball of dough at a time into a 10" long rope on wax paper.
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Cut off 1/4" thick rounds, place rounds 1" apart on the baking sheets and bake 9-10 minutes per batch.
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Carefully remove to wire racks to cool (be careful as they can be brittle).
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To prepare filling, melt chocolate and butter over low heat in a small pan, stirring often.
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Cool 5 minutes, then add the sour cream and stir to combine.
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Gradually add the powdered sugar, stirring until mixture is spreadable.
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Spread 1/2 tsp of the filling on a cooled cookie, then top with another cookie, making a sandwich.