Preheat the oven to 350 degrees F. Place the chocolate in a medium bowl over a pot of simmering water and melt completely. Sift together the flour, baking powder and salt and set aside. Beat the sugar and butter in the bowl of an electric mixer. Add the eggs, then the espresso and vanilla, and beat on high speed, 2 minutes. On low speed, beat in the melted chocolate mixture until just combined. Fold in the flour mixture with a rubber spatula until just combined. Stir in the chocolate chips and pecans. Scoop the mixture onto a parchment-lined baking sheet using a small ice-cream scoop , putting 8 cookies on each sheet. Bake 10 to 11 minutes (and not any longer). The surface will be dry but the inside will still be soft. Cool on a baking sheet. These cookies are best eaten the day they are made. Wrap leftovers well in plastic wrap and store in an airtight container.