Preheat your oven to 325F. Line an 8x8 inch baking pan with parchment paper with a slight overhang on each side. This will make it easier for you to lift up the bars later.
In a medium bowl, stir 1 cup all-purpose flour, ½ tsp baking powder and ½ tsp salt. Set aside.
In another larger bowl, stir ⅓ cup melted unsalted butter and ¾ cup brown sugar until incorporated. Add 1 egg, 1 egg yolk and 1 tsp vanilla extract and continue stirring until they’re thoroughly incorporated too.
Add your dry ingredients and M&M chocolate to your wet ingredients and gently stir just until combined. Do not over mix.
Transfer the batter to your prepared pan and smoothen the top with a spatula. Optional if you’re feeling extra Christmasy: sprinkle more M&Ms on top.
Bake for 20-30 minutes or until the top starts to crack and the sides start pulling away from the pan. It will look a little underdone but don’t worry, the bars will continue to set as they cool in the pan.
Allow the bars to cool in the pan completely before lifting them up and slicing them. I usually slice them into 16 squares but you can slice them smaller or bigger if you like.