Preheat The Oven & Prep The Pan. Start by preheating the oven to 325 degrees F. Line an 8x8" metal pan with parchment paper and set aside.
Melt The Butter, Oil, Sugar & Chocolate. In a medium saucepan, melt butter, oil, sugar and 2/3 cup chocolate chips over low heat. Stir until the chocolate chips are melted and remove from heat. (The sugar will likely still look a bit grainy. This is fine!)
Beat In The Eggs & Egg Yolk. Whisk in the eggs and egg yolk, one at a time, stirring to fully incorporate before adding the next egg/yolk.
Add The Vanilla. Mix in the vanilla extract.
Stir In The Dry Ingredients. Add the gluten-free flour blend, cocoa powder, salt, and baking powder. Stir until just combined.
Fold In The Chocolate Chips. Add the second 2/3 cup chocolate chips and fold until just evenly distributed. Transfer the brownie batter to your prepared pan and smooth the surface evenly.
Sprinkle With M&Ms. Add the M&Ms to the top of the brownie batter, doing your best to distribute them evenly.
Bake & Cool. Pop the brownies in the oven and bake for 25-30 minutes or until the surface no longer jiggles in the center and the brownies look set around the edges. (Try not to overbake, as this will dry the brownies out a bit.) Let the brownies cool at least 30 minutes before slicing. I recommend cooling the brownies completely for the neatest cuts.
Slice & Enjoy. Cut into slices (9 large squares, 12 rectangles, or 16 small squares) and enjoy. Store leftover M&M brownies in the fridge 2-3 days, or in a freezer-safe bag or airtight container in the freezer up to 2 months.