Fill a small saucepan with about an inch of water and bring to a gentle simmer. Set a heatproof bowl on top, making sure it doesn't touch the water.Add the sweetened condensed milk, butter, and white chocolate chips to the bowl. Stir occasionally until fully melted and smooth, about 5 minutes. Fill a small saucepan with about an inch of water and bring to a gentle simmer. Set a heatproof bowl on top, making sure it doesn't touch the water.
Add the sweetened condensed milk, butter, and white chocolate chips to the bowl. Stir occasionally until fully melted and smooth, about 5 minutes.
Remove from heat. Stir in the sugar cookie mix and most of the nonpareils quickly, just until combined. Do not overmix.
Line a 9×5-inch loaf pan with parchment paper. Pour in the fudge and spread evenly. Top with reserved nonpareils.
Refrigerate for at least 3 hours until fully firm.
Lift out by the parchment overhang, place on a cutting board, and slice into 1-inch squares with a warm, dry knife.