01 - Preheat oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment paper, ensuring an overhang for easy removal.
02 - In a large mixing bowl, whisk together melted butter, granulated sugar, and brown sugar until well combined and smooth.
03 - Beat in the eggs one at a time until fully incorporated, then stir in the vanilla extract.
04 - Sift the flour, cocoa powder, and salt into the wet ingredients. Gently fold until just combined, taking care not to overmix.
05 - Gently fold in half of the M&Ms and half of the chocolate chunks into the batter.
06 - Pour and spread the batter evenly into the prepared baking pan.
07 - Distribute the remaining M&Ms, chocolate chunks, mini marshmallows, and optional nuts evenly over the batter. Use a knife to gently swirl the toppings for a marbled effect.
08 - Bake for 28–32 minutes, or until the center is set but retains a fudgy texture. Avoid overbaking.
09 - Remove the pan from the oven. Sprinkle with festive sprinkles. Allow the slab to cool completely in the pan before lifting and slicing into desired portions.