In a pan heat the butter and brown sugar over medium heat to until incorporated well and the sugar starts of dissolve. Stirring constantly.
Remove from heat and let cool slightly if needed. Stir in the eggs and vanilla.
Add the flour, baking powder, and baking soda. Mix well.
Spread mixture into a greased 9 by 13-inch pan.
Sprinkle the M&Ms on top.
Bake in a 350-degree oven for 30 minutes or until done.
Cool slightly, then cut into bars while warm.