Preheat the oven to 350°F and grease and line an 8×12 or 9×11 baking pan with parchment paper.
In a large bowl, mix the melted butter, oil, granulated and brown sugar. Whisk until combined.
Add vanilla and eggs, beating for 30 seconds until the batter lightens.
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, avoid overbaking. If you prefer cakey brownies, bake for 5-8 minutes longer.
Remove from the oven and let cool completely before frosting.