Line an 8×8 baking pan with parchment paper and set aside. This prevents the fudge from sticking.
In a heat-safe medium bowl, place 1 cup semi-sweet chocolate chips, 1 tablespoon unsalted butter, and 7 ounces sweetened condensed milk. Microwave in 30-second increments, stirring until melted and smooth.
Stir in 1 teaspoon vanilla extract. Spread the mixture evenly into the prepared pan. Chill in the fridge for 15 to 20 minutes until set.
For the white chocolate layer, melt 1 cup white chocolate chips, 1 tablespoon unsalted butter, and 7 ounces sweetened condensed milk in the same way. Stir in another 1 teaspoon vanilla extract.
Pour the white chocolate mixture over the set layer, spreading it evenly. Immediately sprinkle with 2 tablespoons Christmas nonpareils.
Cover and refrigerate until set, about 2 to 3 hours or overnight for best flavor. Once set, lift out using parchment and cut into squares.
