1. Preheat the oven to 350°F and line a 9×13-inch baking pan with parchment paper or grease it lightly.

2. In a large bowl, beat the softened butter, brown sugar, and granulated sugar until creamy and fluffy.

3. Add the eggs one at a time, then mix in the vanilla extract until smooth.

4. Stir in the flour, baking soda, and salt just until the dough comes together.

5. Fold in the semisweet chocolate chips, white chocolate chips, most of the M&M’s, crushed pretzels, and mini marshmallows.

6. Press the dough evenly into the prepared pan.

7. Sprinkle the remaining M&M’s, a few extra pretzel pieces, and the holiday sprinkles over the top, pressing them in lightly.

8. Bake for 26 to 32 minutes, until the edges are golden and the center is just set.

9. Let the bars cool completely in the pan before slicing into squares.