Preheat oven to 350°F Butter and flour an 8x8 or 9x9-inch non stick baking pan or spray with baking spray. Set aside.
In a large mixing bowl, using an electric mixer cream together butter, granulated sugar, brown sugar and vanilla extract.
Add eggs one at a time beating well after each addition.
In a medium size mixing bowl use a whisk to sift together flour, baking powder and salt.
Lower the speed of the mixer and gradually add the sifted dry ingredients until all is used. Beat just until fully combined, stopping to scrape the sides of the bowl as needed.
By hand, mix in ½ cup of M&M’s. Spread batter evenly into prepared pan then sprinkle the top with the reserved ½ cup M&M’s gently pressing into the batter. .
Bake for 25-30 minutes or until a toothpick inserted into the center comes back showing moist crumbs. Cool completely in the pan on a cooling rack.
Store in an airtight container at room temperature for up to 3 days.