Preheat the Oven : Preheat your oven to 350°F (175°C). Grease and line a 9x13-inch baking pan with parchment paper for easy removal.

Prepare the Dough : In a large mixing bowl, combine the melted butter, brown sugar, and granulated sugar. Mix until smooth and creamy. Add the eggs and vanilla extract. Whisk until fully combined.

Combine Dry Ingredients : In a separate bowl, whisk together the all-purpose flour, baking soda, and salt. Gradually add the dry ingredients to the wet ingredients, mixing just until combined. Be careful not to overmix.

Fold in the Festive Add-Ins : Gently fold in the red and green M&Ms and white chocolate chips, reserving a few M&Ms to sprinkle on top for decoration.

Assemble and Bake : Spread the dough evenly into the prepared baking pan, smoothing the top with a spatula. Sprinkle the reserved M&Ms on top for a festive look. Bake in the preheated oven for 20-25 minutes, or until the edges are golden brown and a toothpick inserted in the center comes out clean.

Cool and Slice : Allow the cookie bars to cool completely in the pan before slicing into squares or rectangles. Serve and enjoy the festive flavors of Christmas!