Preheat oven to 325 degrees. Line a 9x13 baking dish with nonstick foil or spray with baking spray.
Add flour, cocoa powder, baking powder, baking soda and salt in a bowl and whisk to combine. Set aside.
In a large saucepan or pot, melt butter over medium-low heat. Add chocolate chips and stir until melted. Remove from heat and whisk in granulated and brown sugar. Next, whisk in milk then eggs and vanilla. Whisk until mixture is smooth.
Gently whisk in flour mixture until just combined. Pour batter into prepared pan and bake 40-45 minutes or until toothpick inserted comes out with a few crumbs. Let cool completely.
Make buttercream by adding butter to a stand mixer (or an electric hand mixer) then whip on high speed for 5-7 minutes. Once the butter is pale in color, slowly mix in sugar a little at a time.
Add vanilla, peppermint extract, and milk. Whip for 3-4 minutes. Mix in ¼ cup crushed peppermint candies.
Spread buttercream on top of cooled brownies then sprinkle remaining ½ cup crushed candies on top.