Preheat oven to 375 degrees. Spray half size baking sheet with cooking spray and set aside. In a large bowl with a handheld electric mixer or a stand mixer, beat butter, sugars, egg and vanilla until fluffy, about 2-3 minutes. Add the dry ingredients and mix until combined. Add 1 3/4 cup chocolate chips, 3/4 cup butterscotch chips and 3/4 cup M&M's and mix together. Press into the prepared pan and sprinkle reserved chips and M&M's over the dough and gently press in. Bake for 18-20 minutes until light golden brown. Don't overbake! These bars are definitely better underbaked by just a few minutes rather than being overbaked so watch them carefully. Let cool completely on a wire rack before slicing into squares.