Preheat the oven to 325 degrees and line an 8x8 or 9x9 pan with parchment paper.
Combine the semi-sweet chocolate, butter, and olive oil in a small saucepan and melt it over medium-low heat, stirring constantly until creamy. Set it aside while you make the rest of the batter.
Whisk the cocoa powder, flour, protein powder, and salt in a medium bowl.
Beat the eggs, brown sugar, granulated sugar, and vanilla in a large bowl until combined. Then, add the melted chocolate and mix again.
Add the dry ingredients to the wet ingredients and mix with a rubber spatula until just combined.
Fold in the chopped chocolate bar and M&Ms, giving 2 to 3 mixes to combine without overmixing.
Pour the batter into the prepared pan and spread it out evenly. Then, bake it for 28 to 38 minutes until a toothpick inserted into the center comes out almost clean.
Cool the brownies in the pan on a wire cooling rack for at least an hour. The top will begin to crack more. Remove the brownies once cooled and slice.
Notes
Storage
Store these brownies at room temperature for up to a week or in the freezer for up to 2 months, as specified in the post. Never refrigerate the brownies or they'll get dry.
To ensure your brownies stay as moist as possible, add a slice of white bread to the container in which they're stored. The moisture from the bread will keep the brownies from drying out.
Tips
Mix your brownie batter until just combined to ensure the most tender brownies possible. The batter may be lumpy, but as long as there are no visibly dry ingredients, it's fine.
If you add coffee to the brownies, make it very strong so you don't have to add much liquid. You don't want to throw off the ratios.