Prepare the Pastry: In a food processor, pulse the flour and cold butter until the mixture resembles coarse crumbs. Add the powdered sugar. Add the egg yolk and ice water, pulsing until the dough just comes together. Form into a disc, wrap in plastic, and chill for at least 30 minutes.

Roll out the dough and line a 9-inch tart pan with a removable bottom. Trim the edges. Prick the bottom all over with a fork (blind bake prep). Freeze the shell for 15 minutes.

Blind Bake the Crust: Preheat oven to 375°F (190°C). Line the chilled crust with parchment paper and pie weights. Bake for 15 minutes. Remove weights and paper, and bake for another 5-7 minutes until lightly golden. Let cool slightly.

Make the Caramel: In a medium saucepan over medium heat, combine the 1 cup of sugar and 1/4 cup of water. Swirl gently (do not stir) until the sugar dissolves. Cook until the mixture turns a deep amber color. Remove from heat immediately and carefully whisk in the warmed heavy cream, followed by the 4 tablespoons of butter and sea salt. Whisk until smooth. Stir in the vanilla extract.

Assemble and Bake: Pour the warm caramel evenly into the bottom of the pre-baked tart shell. Arrange the thin nectarine slices decoratively over the caramel, slightly overlapping them in concentric circles if desired.

Bake the tart at 375°F (190°C) for 30–35 minutes, or until the nectarines are tender and the caramel is bubbly. Allow the tart to cool completely on a wire rack before carefully unmolding and serving.