Line an 8x8-inch square baking pan with parchment paper, leaving some overhang on two opposite sides to form a sling. Lightly grease the parchment with butter or nonstick spray.

In a large microwave-safe bowl, add the white chocolate chips, sweetened condensed milk, and butter pieces. Stir just enough to coat the chips so they melt more evenly.

Microwave the mixture on 50% power for 1 minute, then remove and stir well. Continue microwaving at 50% power in 20–30 second bursts, stirring after each burst, until the mixture is mostly smooth. Stop when only a few small lumps remain; the residual heat will melt them as you stir.

To the warm mixture, add 1/2 cup thick caramel sauce, vanilla extract, and fine sea salt. Stir gently until fully combined and no caramel streaks remain, or leave it slightly under-mixed for a marbled effect if you prefer.

Quickly pour the fudge mixture into the prepared 8x8-inch pan. Use a rubber spatula to spread it into an even layer, pressing into the corners and smoothing the top. If it is very sticky, lightly grease the spatula or back of a spoon.

Warm 2–3 tablespoons of thick caramel sauce in the microwave for about 10–15 seconds, just until pourable. Drizzle it over the surface of the fudge in thin lines or squiggles. Use a toothpick or butter knife to gently swirl the caramel into the top layer without going too deep. Sprinkle flaky sea salt evenly over the top.

Cover the pan loosely with plastic wrap or foil, avoiding contact with the surface. Refrigerate for at least 2–3 hours, or until the fudge is firm enough to cut cleanly. For the neatest edges, chill overnight.

Use the parchment sling to lift the chilled fudge out of the pan and onto a cutting board. With a long, sharp knife, cut into about 1-inch squares (or larger if desired), wiping the knife with a warm, damp cloth between cuts. Serve slightly chilled or at cool room temperature.