Preheat oven to 350 degrees. Line a 9x13 pan with tin foil, letting the edges hang over the side of the pan. Spray with cooking spray and set aside.
Break baking chocolate into chunks and slice butter into 1/2 in. pats. Put both into a microwave-safe bowl and microwave for 30 seconds increments, stirring after each time until smooth. Set bowl aside and stir every now and then to keep smooth.
In a stand mixer with paddle attachment, beat eggs, sugar, and vanilla for 2 minutes. While it is mixing, whisk flour and baking powder together in a separate bowl. With the mixer running, slowly add the melted chocolate and beat to combine. Turn mixer to low and add in flour mixture by the spoonful, mixing just until combined. Pour batter into pans lined with foil and bake for 20-30 min. or until a toothpick inserted into the center comes out clean.
Cool brownies to room temperature on a cooling rack. Prepare frosting by beating all frosting ingredients, starting with 1 1/2 Tablespoons of milk and adding more if needed. (I only used 1 1/2 T.) Beat until light and fluffy, then spread evenly over brownies. Chill in refrigerator.
While the brownies are in the fridge, make the chocolate glaze. In another (or the same) microwave-safe bowl, combine the chocolate chips and 6 Tablespoons butter. Microwave in 30 second increments, stirring in between, just until melted and smooth. Let cool for about 15 minutes, then spread on top of frosted brownies. Be gentle, careful not to pull up the frosting below. Sprinkle with peppermint baking chips and return to refrigerator to cool.
When the chocolate glaze has hardened, lift brownies from pan using foil overhang. Slice into squares and eat, serve, or ship. Enjoy!