Preheat oven to 350 degrees. Spray an 8x8 square baking pan with non-stick cooking spray. Line the bottom of the pan with parchment paper and then spray the parchment with non-stick cooking spray.
In a microwave safe bowl, add in the butter and milk chocolate chips. Microwave for 1 minute. Stir with a spatula. Microwave 30 seconds and stir again. The chocolate should be melted and completely smooth. Let cool for 10 minutes.
Add in the granulated and brown sugar, eggs, and vanilla. Using a hand mixer, mix for 2 minutes until the mixture turns pale and becomes thick, light, and fluffy.
Add in the cocoa powder and flour and mix on low until almost incorporated. Turn up to medium and beat for about 30 seconds.
Pour and spread evenly in the baking pan. Bake for 35-38 minutes or until done. Place a toothpick in the center of the brownies and if it comes out clean or with moist crumbs it's done. If it still has batter on it, it needs a little more time.
Once done, remove from oven and let cool completely. You can place it in the fridge to cool down faster.
Remove from baking pan and set aside.
Peppermint Frosting
In a medium bowl, add in the butter and powdered sugar. Beat on low until most of the sugar is incorporated.
Add in the peppermint oil and milk. Start with 2 tablespoons and add the additional if your frosting is still too think. Mix on high until light and fluffy about 1-2 minutes.
Stir in the peppermint crunch with a spatula.
Spread the frosting evenly onto the top of the brownies. Make sure to get all the way to the edges and corners.
Place in the fridge for about 30 minutes and prepare ganache so it has time to thicken slightly.
Ganache
In a microwave-safe bowl, add in the heavy cream and milk chocolate chips. Place in the microwave for 30 seconds. Stir with a spatula. Place back in the microwave for another 30 seconds and stir. It should look like melted chocolate and completely smooth. Set aside and let cool until it thickens slightly.
Remove brownies from the fridge and spread the ganache over the peppermint frosting. Be sure to get all the edges and corners. If the ganache is too thick to pour on brownie, warm it up in the microwave for about 15-20 seconds.
Place any sprinkles or decorations on top at this point.
Place back in the fridge to cool completely. It should take about an hour.
Trim off the brownie edges and slice into squares.
Notes
If you don't have peppermint oil such as LorAnn's Oils, use about ¾ tsp. to 1 tsp. of peppermint extract. The oils are much stronger than the extracts.