Preheat the oven to 350°F. Grease and line an 8x12 or 9x11 baking pan with parchment paper.

In a large bowl, combine melted butter, oil, granulated sugar, and brown sugar. Whisk to combine.

Add vanilla and eggs, and beat for 30 seconds until the batter becomes a lighter brown.

Sift in flour, cocoa powder, and salt. Mix lightly until just combined. Gently fold in the chocolate chips and M&Ms.

Pour the batter into the prepared pan and spread evenly. Bake for 25-30 minutes, or until the center no longer jiggles. For fudgy brownies, do not overbake. For cakey brownies, bake for 5-8 minutes longer.

Remove from the oven and let cool completely before frosting.

In a large bowl, combine melted butter, cocoa powder, and vanilla extract. Beat with a hand mixer or stand mixer until smooth.

Sift the powdered sugar and add it to the bowl slowly until incorporated. Beat on medium speed until the frosting is smooth and creamy, about 1 minute.

Add 1 tablespoon of milk at a time until you reach the desired consistency.

Spread the frosting on the cooled brownies and sprinkle with the remaining M&Ms. Enjoy!