1. Line an 8×8-inch pan with parchment paper, leaving enough overhang to lift the fudge out later.

2. Mix the chocolate sandwich cookie crumbs with 2 tablespoons melted butter until evenly coated, then press the mixture firmly into the bottom of the pan.

3. In a medium saucepan over low heat, combine the white chocolate chips, sweetened condensed milk, and 2 tablespoons unsalted butter. Stir until smooth and melted.

4. Add the heat-treated cake mix, vanilla extract, and salt. Whisk until the mixture is thick, creamy, and fully combined.

5. Remove the pan from the heat and fold in most of the red and green candy pieces, saving a little for the top.

6. Spread the fudge mixture evenly over the chocolate cookie base.

7. Sprinkle the remaining candy pieces and the mini marshmallows or white candy bits over the top, then gently press them in.

8. Refrigerate for at least 3 hours, or until fully firm.

9. Lift the fudge from the pan, cut into squares, and serve chilled or slightly cool.