Place racks in the upper third and middle of the oven and preheat to 325 degrees F. Line two cookie sheets with parchment paper or a silpat mat. Set aside.
Bring 1 inch of water to a gentle simmer in the bottom of a double boiler (ensure that the water is just barely simmering). Place semi-sweet chocolate and butter in the top of the double boiler. Gradually heat and stir, just until the butter melts. Remove from simmering water and stir until chocolate melts completely. The chocolate may be thick—this is OK, but ensure the chocolate does not smell or taste burned. Set aside to cool.
In a large bowl, stir together the flour, baking powder, and salt. Set aside.
Once the chocolate has cooled slightly (it should be warm but not hot), whisk in the granulated sugar, espresso powder, and vanilla (if the chocolate is very thick, you may need to use a mixer.)
Mix in the eggs one at a time, ensuring that you incorporate well after each.
Add the chocolate mixture all at once to the dry ingredients, then fold in with a wooden spoon, until the flour is nearly incorporated. The batter will be very thick.
Fold in the walnuts and dark chocolate chips.
Scoop cookies by tablespoons (take care to measure the dough to ensure proper baking) onto prepared baking sheets.
Bake for 11 minutes, switching the pans on the racks and rotating each 180 degrees halfway through (the cookies will look slightly underdone.) Let rest for 5 minutes on the baking sheet, then remove to a wire rack to let cool completely...or for as long as you can wait.
TO MAKE COOKIE DOUGH AHEAD: Cookie dough can be covered in plastic wrap and refrigerated in an airtight storage container for up to 3 days or frozen for up to 1 month.
TO STORE: Store baked cookies in an airtight storage container for up to 5 days. Separate cookie layers with a piece of wax paper.
TO FREEZE: Freeze cookies in an airtight, freezer-safe storage container or ziptop bag for up to 3 months. Thaw and enjoy as desired.