Begin by lining a 9x9-inch baking pan with parchment paper. Leave some overhang on the sides to make it easier to lift the fudge out later.
In a microwave-safe bowl, combine the chopped white chocolate and sweetened condensed milk. Microwave the mixture in 30-second intervals, stirring in between, until the chocolate is completely melted and smooth.
Once your chocolate mixture is smooth, stir in the vanilla extract and sea salt.
If you’re using rainbow sprinkles, fold them into the fudge mixture now.
Pour the fudge mixture into the prepared baking pan, spreading it evenly with a spatula.
Refrigerate the fudge for at least 2 hours or until it is firm.
Using a sharp knife, cut the fudge into small squares and serve.
