Preheat oven to 350°
Butter and flour a half sheet pan(18x13) or use Bakers Joy.
In a large glass bowl over a pan of simmering water melt 1 pound of butter, one pound of chocolate chips, and 6 oz of unsweetened chocolate, chopped. Remove from heat and allow to cool slightly. In a medium bowl stir together eggs, coffee granules, vanilla, mint extract and sugar. Add to the chocolate mixture and allow to come to room temperature.
In a medium bowl, add 1 ¼ cups flour, baking powder, and salt and whisk together. Add to the cooled chocolate mixture and stir until just combined.
Unwrap the Andes Mints and chop into small pieces. Add to a small bowl along with 8 oz of mini chocolate chips. Add 1 tbsp of flour and toss to coat. Add to the cooled batter and fold in until evenly distributed. Pour onto prepared half sheet pan and put into preheated oven.
Bake for 20 minutes, then carefully lift the pan a few inches and drop onto the oven rack to allow any air trapped under the brownies to escape. The brownies will fall slightly. Bake for another 15-18 minutes or until a toothpick inserted comes out with just a few moist crumbs. Allow to cool completely before decorating.