Set up a double boiler and heat to a gentle simmer.
Combine sweetened condensed milk, butter, and white chocolate chips in the top of the double boiler. Stir mixture occasionally until chips are completely melted.
1 (12-ounce) can sweetened condensed milk, 2 tablespoons salted butter, 2 ¾ cups white chocolate chips
Remove the melted white chocolate mixture from the heat. Add sugar cookie mix, and nonpareils, reserving 2-3 teaspoons of the sprinkles to sprinkle on top. Mix together quickly (the sprinkles will bleed more into the fudge mixture the longer it takes to mix).
1 cup sugar cookie mix, ⅓ cup nonpareils
Pour into a plastic wrap, or parchment paper lined 9x5 inch loaf pan for thick fudge, or 8x8 in pan for thinner fudge. Sprinkle with reserved nonpareils.
Place in refrigerator for at least three hours, until fudge is set.
Remove from refrigerator, and turn onto a cutting board, remove plastic wrap, and cut into roughly one inch pieces.