Preheat oven to 350 degrees. Coat a 9 x 13 inch baking pan with cooking spray and parchment paper; set aside.
In a large bowl, use a handheld mixer to beat together the butter, brown sugar, eggs, and vanilla until fluffy; about 2 minutes. Mix in the flour, baking powder, and salt until well combined. Fold in the chips and ½ cup of the M&M candies.
Pour the mixture into the pan using a rubber spatula to spread. Press the remaining M&Ms on top of the bars. Bake for about 25 minutes, or until the bars turn golden brown and the edges start to pull away from the sides of the pan. Be careful not to over cook the bars.
Carefully press the candy eyeballs into the top of the bars. Cool in pan on a wire rack then transfer to refrigerator to cool completely before removing from pan.
Notes
Lining your baking sheet with parchment will make the bars easier to remove from the pan and easier to cut into squares.
This recipe also works for cookies. Just roll the dough into balls and bake for about 10 minutes.
You can press the candy eyes into the batter before baking. However, there is a chance the eyes will brown slightly so I recommend adding them to the bars right out of the oven instead. Press them in using a toothpick to prevent burning yourself.
Store leftover bars in an airtight container in the refrigerator for up to a week, or freezer for up to a month.