Preheat the oven to 350 degrees F. Spray a 9x13-inch baking pan with nonstick cooking spray.
In a large mixing bowl, whisk together 1 cup melted butter, 4 large eggs, 1 cup granulated sugar, 1 cup brown sugar, and 2 teaspoons vanilla extract until smooth and well combined.
Add ? cup cocoa powder, 1 cup all-purpose flour, ½ teaspoon salt, and ½ teaspoon baking powder to the wet ingredients.
Stir gently by hand just until combined, being careful not to over mix.
Fold in 1 cup of the Christmas M&M's.
Spread the brownie batter evenly into the prepared baking pan.
Bake for 20-25 minutes, or until a toothpick inserted into the center comes out with just a few moist crumbs. Be careful not to over bake.
Allow the brownies to cool slightly while preparing the ganache.
In a small saucepan, heat ½ cup heavy whipping cream over medium heat until it just begins to simmer.
Remove from the heat, add 2 cups semi-sweet chocolate chips, and stir until completely melted and smooth.
Pour the ganache over the brownies and spread it into an even layer with an offset spatula or the back of a spoon.
Sprinkle the remaining 1 cup Christmas M&M's evenly over the ganache, then top with Christmas sprinkles.
Refrigerate the brownies for at least 1 hour, or until the ganache is firm.
Cut into 16 squares, serve, and enjoy!
Notes
Store the leftover brownies in an airtight container at room temperature for up to 1 week.