Line an 8×8 baking pan with parchment paper and set aside.
Place the semi-sweet chocolate chips in a heat-safe medium bowl and add 1 tablespoon of butter and ¾ cup of the sweetened condensed milk (half a can). Microwave in 30-second increments, stirring in between each interval, until the chocolate is fully melted. Once the chocolate is melted, stir in 1 teaspoon of vanilla extract.
Spread the mixture into the prepared baking pan. Allow that mixture to chill in the refrigerator for 15-20 minutes, or until the chocolate has set up.
Once the semi-sweet chocolate layer of fudge has been set up, prepare the white chocolate layer. Place the white chocolate chips in a heat-safe medium bowl and add 1 tablespoon of butter and ¾ cup of the sweetened condensed milk (the remaining amount in the can). Microwave in 30-second increments, stirring in between each interval, until the chocolate is fully melted. Once the chocolate is melted, stir in 1 teaspoon of vanilla extract.
Pour the white chocolate layer over the chocolate layer of fudge, using an offset spatula to spread if needed. Immediately sprinkle the nonpareils over the white chocolate layer.
Cover the pan with a lid or plastic wrap and refrigerate until the fudge is set, 2-3 hours. I find overnight is best, in terms of flavor and firmness of the fudge.
Once set, remove the fudge, using the parchment paper from the pan cut it into 1-inch pieces.
Notes
Storage:Store the fudge in an airtight container in the refrigerator for up to 2 weeks. I don't recommend freezing Christmas Fudge. Sprinkles: Feel free to use any holiday sprinkles you like. Or use crushed candy canes for a peppermint twist on this Christmas Fudge. Christmas Fudge without Layers: Instead of layering white chocolate over semi-sweet chocolate chips, feel free to use all semi-sweet chocolate or all white chocolate. Melt the chocolate chips of your choice with the full can of sweetened condensed milk and 2 tablespoons of butter until melted. Pour into the prepared baking dish and top with sprinkles of your choice.