Preheat oven to 350F. Line an 8-by-8-inch baking pan with aluminum foil, spray with cooking spray; set aside.
In a large, microwave-safe bowl melt the butter, about 1 minute on high power.
Wait momentarily before adding the egg so you don’t scramble it. Add the egg, brown sugar, vanilla, and whisk until smooth.
Add the flour and stir until just combined, don’t overmix.
Stir in 1 cup M&Ms
Turn batter out into prepared pan, smoothing the top lightly with a spatula.
Add remaining M&Ms to top of batter, dispersing them evenly, and pressing them down lightly with your finger.
Bake for about 23 minutes, or until done. At 23 minutes (shown in photos) they’re very soft and moist; if you like firmer and dryer bars, bake for a few minutes longer. A toothpick inserted in the center should come out clean, or with a few moist crumbs, but no batter.
Allow bars to cool in pan for at least 30 minutes before slicing and serving.
Notes
Storage: Bars will keep airtight at room temperature for up to 1 week, or in the freezer for up to 6 months.