Preheat oven to 350℉. Line an 8 x 8-inch baking pan with aluminum foil and spray with cooking spray. Set aside.
In a small saucepan, melt butter and allow to cool briefly.
In a medium bowl place the egg, brown sugar, and vanilla. Drizzle melted butter and whisk until smooth.
Add the flour and salt and stir until just combined.
Stir in the chocolate chips and M&M's.
Place the batter into prepared pan and spread to edges, smoothing the top with a spatula.
Bake at 350℉. for 25 to 30 minutes or until done. When a toothpick inserted in center has crumbs, they are done. Allow bars to cool at least 1 hour in pan before removing and cutting. Bars will firm up upon cooling.