Line an 8-inch square baking pan with parchment paper or foil, leaving enough overhang on two sides to lift out the finished fudge.

Place the semisweet chocolate chips and sweetened condensed milk in a microwave-safe bowl. Microwave at 50% power in 30-second intervals, stirring after each interval, until smooth; this usually takes 2 to 3 minutes.

Stir the vanilla extract into the warm chocolate mixture, then fold in about half of the chopped pecans. Mix gently just until the pecans are distributed.

Spread roughly half of the fudge mixture in the prepared pan. Drizzle the caramel sauce over the chocolate layer, warming it briefly first if it is too thick to pour.

Cover the caramel with the remaining fudge mixture and smooth the top with a spatula. Sprinkle on the remaining pecans, then add the optional flaky sea salt.

Lightly swirl any visible caramel into the surface with a knife, using only a few gentle strokes. Do not add extra caramel, because too much moisture can leave the fudge soft.

Refrigerate the pan for 1 to 2 hours, or until the fudge is firm. Lift it out using the parchment or foil overhang, then cut it into 16 to 25 small squares.