Line an 8-inch square pan with parchment paper, leaving enough overhang on two sides to lift out the chilled fudge. Before measuring the cookie mix, confirm that it is commercially heat-treated or heat-treat it according to the package or food-safety guidance, and use a mix with no raw egg.

Add the white chocolate chips, sweetened condensed milk, butter, and dry sugar-cookie mix to a saucepan. Warm over low heat, stirring constantly, until the chocolate and butter melt into a smooth mixture; do not let it boil.

Remove the saucepan from the heat and stir in the vanilla extract and optional pinch of salt. Fold in some of the holiday sprinkles only if you want a colorful interior, using just a few gentle strokes so the colors do not bleed.

Immediately scrape the mixture into the prepared pan and spread it into an even layer. Sprinkle the remaining red-and-green decorations over the surface and press them in lightly while the fudge is still soft.

Refrigerate the pan for at least 2 hours, or until the fudge feels firm in the center. Use the parchment overhang to lift it onto a cutting board, then cut it into 36 small squares with a sharp knife.