Preheat the oven to 350°F. Line a 9×13-inch metal baking pan with parchment paper, leaving a little overhang for easy removal.
In a large mixing bowl, beat the softened butter and brown sugar together until smooth and well combined.
Mix in the eggs and vanilla extract until the mixture is creamy and fully blended.
In a separate bowl, whisk together the flour, salt, and baking powder.
Gradually add the dry ingredients to the wet mixture, mixing just until combined. Do not overmix.
Gently fold in the Christmas M&M’s and sprinkles.
Press the dough evenly into the prepared pan. Bake for 22–25 minutes, or until the edges are set and the center is just baked through.
Allow the cookie bars to cool completely before cutting into squares.