Preheat oven to 300° F. Line a large (14 inch) round pizza pan with a circle of parchment or non-stick foil.
Mix flour, baking soda and salt together; set aside.Cream butter, shortening and both sugars by hand or with an electric mixer. Beat in the eggs and vanilla. When well mixed, add flour mixture and stir until blended Fold in chocolate chips.Put all the batter in the pan and press it into a large circle. Stop the circle about 1 ½ to 2 inches from the rim. Press M&M’s decoratively into top.
Bake on center rack for 32-35 minutes or until cookie is golden around edges and appears set.