Prepare the pan: Spray a 9×9 pan with cooking spray, then line it with parchment, leaving enough overhang to lift the fudge out easily.
Heat the base: Pour the sweetened condensed milk into a large pot and warm over medium heat, stirring nonstop to prevent scorching.
Melt everything together: Add the white chocolate chips, sugar cookie mix, butter, and salt. Stir until the mixture melts completely and becomes smooth and glossy.
Add flavor and color: Remove from heat, then fold in the vanilla and about 2 tablespoons of sprinkles.
Transfer to pan: Pour the fudge mixture into your lined pan and smooth the top. Add the remaining sprinkles over the surface.
Chill to set: Refrigerate the fudge for at least 2 hours, or overnight, until firm.
Slice and serve: Lift the fudge out using the parchment, cut into 36 squares, and enjoy your Christmas treat.