Preheat oven to 350 degrees. Line an 8x8 baking pan with aluminum foil, spray with cooking spray and set aside.
In a large microwave safe bowl melt the butter, about one minute on high.
Wait momentarily before adding the egg (you don't want to scramble it). Add the egg, brown sugar and vanilla. Whisk until smooth.
Add the flour and stir just until combined, don't overmix.
Stir in the Oreos and M & M's.
Turn batter out into prepared pan, smoothing batter lightly with a spatula.
Bake about 20-22 minutes, or until done, based on add-ins used. The top should be set and the edges should be slightly firm. A toothpick inserted in the center should come out clean or with a few moist crumbs, but no batter. Allow bars to cool at least 30 minutes before slicing and serving. Bars will keep airtight at room temperature for up to one week.