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In a metal bowl set over a saucepan of simmering water melt the unsweetened chocolate, 1½ cups of the chocolate chips, and the butter, stirring until the mixture is smooth, and remove the bowl from the heat.
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In a small bowl stir together the flour, the baking powder and the salt. In a bowl beat the eggs with the sugar until the mixture is thick and pale and beat in the espresso powder and the vanilla.
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Fold the chocolate mixture into the egg mixture, fold in the flour mixture, and stir in the remaining 1½ cups chocolate chips.
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Let the batter stand for 15 minutes. Drop the batter by heaping tablespoons onto baking sheets lined with parchment paper and bake the cookies in the middle of a preheated 350° F oven for 8 to 10 minutes, or until they are puffed and shiny and cracked on top. Let the cookies cool on the baking sheets, transfer them to racks, and let them cool completely.