Pre-heat the oven to 350 degrees. Butter a 9×13 baking dish.
In small saucepan over low heat, melt butter completely. Stir in sugar and continue cooking for 1-2 minutes, stirring constantly. Do not allow sugar mixture to boil.
Pour butter mixture into a large bowl or stand mixer, beat in cocoa powder, eggs, salt, baking powder, espresso powder, and vanilla extract. Mix until well combined.
Stir in the flour and peanut butter M&M's until well combined.
Pour into prepared pan and bake for about 30 minutes, until a tester comes out mostly clean. The edges should be set and the center should still look slightly moist, but not uncooked. Cool on a wire rack.