In a large bowl, combine the brown sugar and butter - mix until combined.
Add in the eggs and vanilla just until blended.
Combine the flour, baking powder and salt; add in a 1/4 cup at a time to the brown sugar mixture.
Stir in 1 cup of M&M's
Spread into a greased 13-in. x 9-in. baking pan.
Add in 1/4 - 1/2 cup more of M&M's to the top.
Bake at 350° for 18-20 minutes or until lightly brown
Cool on a wire rack. Cut into bars.