Melt the bittersweet chocolate pieces in the top of a double boiler or in a heatproof bowl over barely simmering water, stirring occasionally until smooth.

In a large bowl, cream the butter, white sugar, and brown sugar with an electric mixer on medium-high speed until light and fluffy. Reduce the speed to low, and add the eggs and vanilla. Scrape the bottom and sides of the mixing bowl with a rubber spatula.

In a separate bowl, sift together the flour, baking powder, baking soda, salt, and espresso powder. Add half the flour mixture to the creamed butter, mixing well. Add half the melted chocolate and stir until ingredients are well blended. Add the remaining flour mixture, incorporating thoroughly, then add the remaining chocolate. Stir in the chocolate chips and pecans. Cover and refrigerate the dough for 1 hour.

Preheat the oven to 350 deg F. Line two cookie sheets with parchment paper.

Drop the dough in rounded tablespoonfuls onto the prepared cookie sheets, about 2 inches apart.

Bake for 15 to 17 minutes. Immediately slide the parchment paper and cookies onto a wire rack to cool. Store in an airtight container at room temperature for up to a week.