Before proceeding, grease and preheat a 9-by-9-inch baking pan to 350°F.
Melt the butter in a receptacle that is microwave-safe.
Then, incorporate the butter, brown sugar, egg, and vanilla into a mixing basin. Mix thoroughly.
Mix the dry ingredients thoroughly into the basin. After that, transfer the mixture to the baking dish.
Consistently garnish with M&Ms. Bake for twenty to twenty-five minutes.