Preheat the oven to 400°F. Line a baking sheet with parchment.

Unroll the cold puff pastry sheet onto the lined baking sheet, keeping its natural rectangle shape.

Using the tip of a sharp knife, lightly score a border about 3/4 inch in from the edges, all the way around, without cutting through. Prick the center portion a few times with a fork, leaving the scored border untouched so it can puff.

Core the apple, leaving the peel on for color. Cut it in half vertically, then slice into thin, 1/8-inch pieces. A mandoline works well, but a sharp knife is fine too.

Stir together the sugar, cinnamon, and salt in one bowl. In another, mix 2 teaspoons of the melted butter with the vanilla.

Brush the vanilla butter lightly over the center of the pastry. Arrange the apple slices in overlapping rows across the center, keeping the scored border clear.

Brush the apples with the remaining melted butter, then sprinkle evenly with the cinnamon sugar. Brush a little butter over the exposed pastry border too.

Bake 22 to 28 minutes, until the border is puffed and deeply golden and the apples are tender with caramelized edges.

Let the tart cool 5 minutes, then drizzle with the caramel sauce. Finish with a pinch of flaky sea salt, if desired, before slicing into squares. Enjoy!