Fit parchment into a 9x9 baking dish, leaving edges available for lifting, and lightly coat it with non-stick spray. Press the paper into the corners so the fudge can spread into an even square.

Warm the sweetened condensed milk in a large pot over medium heat, stirring frequently. The milk should become hot without developing a browned layer on the bottom.

Add the white chocolate chips, appropriately prepared dry sugar cookie mix, and butter, then stir until fully combined and take the pot off the heat. Look for a thick, glossy mixture without visible chocolate pieces or dry pockets.

Fold a generous amount of Christmas sprinkles into the mixture after removing it from the burner. Use gentle strokes to distribute the color without repeatedly working the sprinkles through the warm fudge.

Spread the mixture across the lined dish and scatter additional sprinkles over the surface. Level the corners and center to create an even thickness for slicing.

Refrigerate for at least 2 hours, or overnight, until firm, then lift out the slab and cut it into bite-sized pieces. The center should hold its shape and the cut edges should remain upright rather than slump.