Preheat oven to 325 degrees. In a large mixing bowl, cream together butter and brown sugar. Add in egg and vanilla and mix well. In a separate bowl, combine flour and baking soda. Add flour mixture to your large mixing bowl and mix until completely incorporated. Fold in M&M's. Pour and spread batter into a greased 9 1/2" pie pan and bake for 25-30 minutes, or until a toothpick comes out clean. Let cool and cut into slices.