Preheat oven to 350 degrees F. Grease 9x13” baking dish.
In medium saucepan, melt the butter and brown sugar, stirring until smooth. When it is smooth, remove from heat and add peanut butter. Allow to cool while combining the flour, baking powder, baking soda, and salt.
Add the eggs to the brown sugar and peanut butter mixture, one at a time, mixing well after each addition.
Add the vanilla and stir to combine.
Pour the mixture into the dry ingredients and stir until combined.
It will be a thick, smooth batter.
Fold in 1 cup of the M&M’s.
Pour into prepared baking dish and top with remaining 1/2 cup M&M’s, pressing them down into the batter.
Bake in preheated oven to 25 minutes. Top will be golden brown.
Let cool completely. Cut into squares and enjoy!