Preheat Oven:

Preheat your oven to 350°F (175°C). Grease or line a 9x13-inch baking pan with parchment paper.

Mix Wet Ingredients:

In a large bowl, whisk together the melted butter, granulated sugar, and brown sugar until smooth. Add the eggs, one at a time, and mix well. Stir in the vanilla extract.

Combine Dry Ingredients:

In a separate bowl, whisk together the flour, baking soda, and salt. Gradually add the dry ingredients to the wet mixture, stirring until just combined.

Fold in Mix-Ins:

Gently fold in the holiday M&M’s, crushed Oreo cookies, and white chocolate chips (if using).

Assemble and Bake:

Spread the batter evenly into the prepared baking pan. Sprinkle with additional M&M’s, Oreo pieces, or sprinkles on top for decoration. Bake for 25-30 minutes, or until the edges are set and a toothpick inserted into the center comes out clean or with a few moist crumbs.

Cool and Slice:

Let the bars cool in the pan for at least 20 minutes before lifting them out using the parchment paper overhang. Slice into 12-16 bars.