Make the crust dough. In a bowl, whisk flour, powdered sugar, and salt. Cut in the cold butter with a pastry cutter or your fingertips until it looks like coarse crumbs. Add the egg yolk and 1 tablespoon ice water. Mix just until it clumps; add more water only if needed. Do not overwork.

Chill and shape. Press the dough into a flat disk, wrap, and chill 30 minutes. On a lightly floured surface, roll into a circle about 1/8 inch thick. Fit into a 9-inch tart pan with a removable bottom. Trim edges, dock the base with a fork, and chill 20 minutes.

Blind-bake the crust. Heat oven to 375°F (190°C). Line the crust with parchment and fill with pie weights or dried beans. Bake 15 minutes. Remove weights and parchment, then bake 8–10 minutes more until lightly golden. Cool completely.

Start the caramel. In a medium, light-colored saucepan, combine sugar and water. Cook over medium heat without stirring until the sugar dissolves and turns clear. Increase heat to medium-high and cook until the syrup turns a deep amber. Swirl the pan gently if needed; don’t stir.

Add butter and cream. Remove from heat. Carefully whisk in butter (it will bubble up), then slowly pour in the warmed cream, whisking until smooth. If lumps form, return to low heat and whisk until dissolved.

Finish the caramel. Stir in vanilla and flaky sea salt. Let the caramel cool 5–10 minutes, until it thickens slightly but is still pourable.

Fill the crust. Pour the warm caramel into the cooled tart shell. Smooth the top. Let it sit at room temperature 30 minutes, then chill 1–2 hours until set but soft.

Optional ganache. Heat cream until steaming. Pour over chopped chocolate, let sit 2 minutes, then stir until glossy. Spread a thin layer over the set caramel. Chill 30–45 minutes to set.

Finish and serve. Sprinkle with a pinch of flaky sea salt just before serving. Slice with a warm, dry knife for clean edges. Serve slightly cool or at room temperature.