Preheat oven to 350 degrees F. Line cookie sheets with parchment paper or Silpat baking liners.
In a double-boiler, melt 8 ounces bittersweet chocolate chunks with the butter, stirring until smooth.
Remove the top of the double-boiler to a heat-proof pad to cool to room temperature, but still liquid. (Stir occasionally as it cools.)
In a small bowl, stir together flour, baking powder, and salt. Set aside.
In a separate large bowl, combine the eggs, sugar, and vanilla extract using a mixer on high speed. Beat until well-combined, light and fluffy, about 3 minutes. Add the cooled chocolate mixture to the egg mixture, mixing on low speed until combined. Add the flour mixture, stirring by hand just until combined but smooth. Fold in the chocolate chips. Refrigerate dough for 30 minutes.
Using a small ice cream scoop, drop tablespoon-sized mounds onto prepared baking sheets, spacing them 1 inch apart. Bake 10 to 12 minutes. Do not overbake. Remove from oven and let rest 5 to 10 minutes before carefully moving to cooling racks using a spatula. Cool completely before storing.
Bittersweet brownie cookies may be made several days in advance. They may also be frozen up to 1 month.
Note: 1-1/2 cups of chopped pecans, walnuts, or peanuts may be substituted for 1 cup of the chocolate chips if desired.