Melt the butter in the microwave in a large glass bowl; set aside.
Spray a 9x13" pan with non-stick cooking spray. (For easier cleanup, line the pan with parchment paper or foil first.) Preheat your oven to 350 degrees.
Add the brown sugar to the butter and whisk until smooth. Whisk in the eggs, peanut butter, and vanilla.
Stir in the flour, baking powder, oats, semi-sweet chocolate chips, 1/2 cup of the milk chocolate chips, and 1 1/4 cups of the M&M's.
Press dough into the prepared pan. Sprinkle the remaining 1/4 cup of chips and 1/4 cup M&Ms on top.
Bake at 350° for 28-35 minutes or until barely set in the middle and golden brown around the edges. For soft & chewy bars, it's better to under-bake rather than over baking.
Let the bars cool for at least 20 minutes, but they cut more cleanly after they have cooled for an hour or two.
Notes
-If you use crunchy peanut butter, you will need to use an additional 1/3-1/2 cup of peanut butter.