Off the top of my head I can't quite remember where I got this recipe, but part of me thinks it was from the M&Ms website. If you're ever looking for baking recipes involving M&Ms their site has some options for you. Happy baking!
Christmas (or Rainbow) Blondies
1 cup butter
1 1/2 cup light-brown sugar
1 tsp vanilla
2 cups flour
1/2 tsp baking soda
1 (12 oz) package of M&Ms
1 cup chopped walnuts or pecans (I chose not to use these)
1. Preheat oven to 350 degrees.
2. Cream butter and sugar until light and fluffy; add egg and vanilla.
3. Combine flour and baking soda; add to creamed mixed just until combined. Dough will be stiff (very stiff in fact).
4. Stir in M&Ms and nuts.
5. Spread dough into greased 9 x 13-inch baking pan (this will be difficult to do with a spoon or rubber spatula due to the stiffness of the dough. In my case is just plopped all the dough into the pan and then spread it out and patted it down with my hands, worked like a charm). Bake 30-35 minutes until wooden toothpick inserted comes out slightly moist with crumbs. Cool completely before cutting.