Lightly spray an 8-inch by 8-inch pan with non-stick cooking spray. Press a piece of parchment in that rises up the sides of the pan on at least 2 sides. Lightly spray the parchment with non-stick cooking spray and set it aside.

Add the sweetened condensed milk, chopped chocolate or chocolate chips, unsalted butter, bourbon, and vanilla to a stainless steel bowl and place it over a saucepan that has 2 inches of boiling water in it. Drop the heat on the burner to low, and stir gently, but constantly until the chocolate has melted and the mixture is smooth. Pour into the prepared pan.

Gently heat up the dulce de leche until it can be poured. I use the microwave for this because it's done very quickly. Drizzle over the top of the fudge and use a butterknife or skewer to swirl the fudge and dulce de leche together. Sprinkle coarse sea salt flakes over the top. Place the pan (uncovered) in the refrigerator for at least 4 hours, but preferably overnight before removing from the pan and slicing into small cubes. Store leftover fudge in the refrigerator for up to 2 weeks.