Preheat oven to 350°F. Line a square 9x9" baking dish with a parchment paper sling or spray with baking spray.
In a large microwave-safe bowl, combine the bittersweet chocolate and butter. Microwave on high in 20-30 second bursts, stirring well between bursts of heat until melted and smooth. Whisk in cocoa powder.
4 ounces bittersweet chocolate, ¾ cup salted butter, ½ cup unsweetened cocoa powder
Add granulated sugar and brown sugar, whisking well to combine. Add eggs and vanilla, beating well either by hand with a whisk or using an electric mixer.
1 cup granulated sugar, ½ cup light brown sugar, 3 large eggs, 2 teaspoons vanilla extract
Sprinkle the flour and salt over the brownie batter and gently stir it in with a wooden spoon or spatula. Stir in chocolate chips and half the M&Ms.
¾ cup all-purpose flour, ½ teaspoon salt, ½ cup chocolate chips, 1 cup M&M's,
Pour the brownie batter into the prepared baking dish and spread into the corners. The batter will be thick. Put the remaining M&Ms on top of the brownie mixture.
Bake for 30 to 35 minutes, until a toothpick inserted into the center of the brownies comes out mostly clean with only a few crumbs attached. Be sure not to overbake.
Cool completely before cutting. Store in an airtight container for up to 5 days on the counter or in the fridge.