Beat the butter and brown sugar until creamy. Add the eggs and vanilla and beat again. Sift together the salt, baking powder, and flour. Slowly beat into the butter mixture. Stir in 1 cup of the M&M candies by hand.
Spread the batter in a greased 9x13 baking dish. Sprinkle the top with the extra 1/2 cup candies. Bake at 350 degrees for 23-25 minutes. Let cool completely before cutting into squares. Makes 24 bars. Store in a sealed container.