Preheat the oven to 350ºF and spray an 8x8 inch glass dish with cooking spray. Set aside.

In a medium saucepan over medium heat, melt the butter and cocoa powder together and stir with a wooden spoon until smooth and glossy. Take off the heat.

Stir in the sugar. Add the eggs and vanilla and continue stirring until fully incorporated. Stir in the flour and salt.

Fold in the milk chocolate and 1/2 cup of M&Ms.* Pour batter into the prepared baking dish and bake for 22-24 minutes or until the top and edges are set and the center isn't jiggly. To test it, insert a toothpick into the center--it should come out with a few moist crumbs--no raw batter. Remove from oven and allow to cool completely.

In a small bowl combine the chocolate and heavy cream. Microwave for 1 minute, remove, and cover with plastic wrap. Let it sit for 5 minutes. Remove plastic wrap and stir until smooth and glossy. Set ganache aside to allow it to cool and thicken.

Using a stand mixer fitted with a whisk attachment, whip the butter until smooth and creamy. Add the powdered sugar and continue whipping, stopping and scraping down sides of bowl to allow everything to blend more evenly. Mixture will appear a little dry.

Add the vanilla, peppermint extract and heavy cream (or milk), and continue whipping the buttercream until light and fluffy, about 1 minute. Spread evenly onto the cooled brownies.

Pour the ganache over the buttercream, tilting the dish slightly to evenly distribute the ganache. Sprinkle with the chopped 1/4 cup of M&Ms and optional sprinkles. Refrigerate for 15 minutes or until ganache has set. Cut into squares and serve.

Brownies will stay fresh wrapped in plastic wrap in the refrigerator for up to 1 week.