Preheat the oven to 350 degrees Fahrenheit.
Line a 9"x13" pan with parchment paper so it hangs over the sides slightly.
In the bowl of an electric mixer, cream together softened butter, brown sugar, granulated sugar, vanilla extract and eggs. Cream it until it lightens in color and gets nice and fluffy.
In a separate bowl, whisk together flour, salt, baking powder and old fashioned oats.
Add the flour mixture to the wet mixture and mix just until combined, scraping down the sides in between.
Stir in milk chocolate chips and 1/2 c. lightly crushed M&M's.
Press/spread dough evenly into prepared pan. Press 1/2 c. whole M&M's on top.
Bake for approximately 18-20 minutes, or just until set. Let cool completely, then cut into bars and serve.