3tablespoonsunsalted butter, cut into 1/2-inch pieces, at room temperature
Flaky sea salt, for sprinkling, optional
Lightly grease an 8 x 8-inch baking pan. Line the pan with parchment paper, allowing the excess paper to overhang the sides, set aside.
In a large, heatproof bowl combine the sweetened condensed milk, vanilla extract, chocolate, and butter; set aside. Fill a saucepan with a little water (you want it about 1/4 full) and bring it to a boil. Reduce the heat to barely a simmer. Place your heatproof glass bowl holding your ingredients on top of the pot. Stir frequently, until the chocolate has melted and the mixture is completely smooth, about 8 minutes.
Remove from heat. The mixture will be very thick. Using a sturdy spatula, scrape the fudge into the prepared pan and smooth the top. Sprinkle with sea salt, if using. Refrigerate for at least 2 hours.
Gently run a butter knife around the edge of the pan to loosen the fudge. Remove the fudge and transfer it to a cutting board.
Peel off parchment paper, then slice the fudge into small pieces. Serve at once or store in the refrigerator for up to 1 month. You may also freeze the fudge, in an airtight freezer safe container for up to two months.