Preheat the oven to 350°F (175°C) and grease a 9x13-inch baking dish.
In a large bowl, mix together the melted butter and sugar until well combined.
Add in the eggs, one at a time, beating well after each addition. Stir in the vanilla extract.
In another bowl, sift together the flour, cocoa powder, and salt. Gradually add the dry ingredients to the wet ingredients and mix until just combined.
Fold in half of the Christmas M&Ms, then pour the batter into the prepared baking dish, smoothing it into an even layer.
Sprinkle the remaining M&Ms on top of the batter.
Bake in the preheated oven for 25 to 30 minutes or until a toothpick inserted into the center comes out with a few moist crumbs.
Allow the brownies to cool in the baking dish before cutting into squares and serving.