1. Preheat oven to 350°F. 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, then remove the bowl from the heat.
2. In a small bowl, stir together the flour, baking powder, and salt. In another bowl, beat the eggs with the sugar until the mixture is thick and pale, then beat in the espresso powder and the vanilla. Fold the chocolate mixture into the egg mixture; fold in the flour mixture, then stir in the remaining chocolate chips. Let the batter stand for 15 minutes.
3. Drop the batter by heaping tablespoons onto baking sheets lined with parchment paper and bake the cookies in the middle of the preheated oven for 8–10 minutes, or until puffed, shiny, and cracked on top. Let the cookies cool on the baking sheets, transfer to racks, and let cool completely.
Makes 36 cookies.
Recipe from the The Gourmet Cookie Book, available for purchase here.