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To prepare the pastry: sift the flour, cocoa and salt together into a bowl.
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Cut the butter into the flour mixture with a pastry blender or 2 knives until it resembles coarse meal.
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Add the sour cream and knead into a ball.
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Divide into 4 equal pieces and refrigerate 2 hours.
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On a floured board, roll each piece of dough out to a 10 x 10 inch square.
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Place each square on a buttered baking sheet, prick all over with a fork and bake in a 400f degree oven 10 minutes.
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Carefully transfer the pastry squares to wire racks to cool.
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They will be very fragile.
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To prepare the filling: Place the chocolate, cream and butter in a saucepan and stir over low heat until the chocolate and butter are melted.
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Beat the egg yolks until thick.
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Beat in a few tablespoons of the hot chocolate mixture and stir back into the hot chocolate.
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Stir in the vanilla and let the mixture cool to spreading consistency.
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Do not refrigerate or it will be too stiff to spread.
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Place 1 pastry layer on a serving plate and spread with the filling.
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Continue layering, leaving the nicest looking pastry square for the top.
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Sift powdered sugar over the top layer.
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Using a pastry bag fitted with a star tube, decorate the top layer with a little of the leftover filling.