Cream butter with electric mixer until creamy and light.
Add sugars and cream again, until light in color.
Add eggs. Combine.
Add vanilla. Combine.
Incorporate flour in 3 additions, just until combined in between additions.
Stir in chopped nuts by hand.
Evenly spread dough into parchment paper-lined 9"x13" pan. Top with M&M's.
Bake in a preheated 350°F oven for 20-25 minutes.
If most of the M&M's sink into dough, garnish with additional candies right after they come out of the oven.
Cool until almost completely cool. Then cut with a very sharp knife into bars or squares.
Video
Notes
Customize cookie bars for holiday with holiday colored M&Ms.General Baking Tips:Always measure dry ingredients with a dry measuring cup. Measure with dry measuring cups or weigh dry ingredients.Always measure liquid ingredients with liquid measuring cupNever break eggs directly into recipe ingredients. Break eggs into a separate bowl, in case you get any shells, so that you can pull them out.Make sure that you know what temperature your oven is set at. Buy an oven thermometer and check oven temperature regularly.Always check baked goods, like cakes or quick breads with cake tester, placed in center. If the tester comes out clean, it’s done.