Preheat oven to 350º. Lightly spray a 10 x 15 jelly roll pay with non-stick spray.
In a large bowl, combine the flour, baking powder and salt. In another large bowl, combine the melted butter and brown sugar, stirring until evenly distributed. Allow to cool slightly.
Add the eggs beating very well. Mix in vanilla. Stir in the flour mixture and when combined add the M & M's.
Spread the batter into the prepared pan. Bake for 24-26 minutes until they are shiny and golden brown on top. Try not to over bake these or they will be dry. A cake tester will most likely not come out clean. Remove from the oven and cool completely before cutting.