Preheat oven to 350° F. Grease a 9 X 13″ baking pan (or line with parchment paper).
In a large microwavable bowl, place semisweet chocolate chips and butter. Cook on high for 2 minutes. Stir mixture with wooden spoon until all of the chips and butter are melted.
In the bowl of a standing mixer, combine the sugar, flour, salt and baking powder. Stir to mix thoroughly.
Add the eggs and vanilla to the chocolate mixture, blending well with a wooden spoon.
Pour this mixture into the bowl with the dry ingredients. Mix on medium speed for 30 seconds, just until ingredients are blended. Stir in the mint chocolate chips and walnuts. Spread into prepared pan.
Bake until the brownie starts to come away from the sides of the pan, about 40-45 minutes.