Add the cold cubed butter to the dry ingredients. Using a pastry cutter or your fingertips, cut the butter into the dry ingredients until the mixture resembles coarse crumbs with some pea-sized pieces.

Stir in the egg yolk and 1 tablespoon cold water until the dough just comes together. If the dough is too dry, add up to 1 more teaspoon cold water. Do not overwork.

Turn the dough onto a lightly floured surface, gather into a disk, wrap in plastic, and chill in the refrigerator for at least 30 minutes (up to 2 days).

Preheat the oven to 350°F (175°C). Lightly butter a 9-inch tart pan with removable bottom.

Roll the chilled dough on a lightly floured surface to about 1/8-inch thickness and transfer to the tart pan. Press gently into the bottom and up the sides, trimming excess. Prick the bottom lightly with a fork.

Line the shell with parchment paper and fill with pie weights or dried beans. Blind bake for 15 minutes, remove weights and parchment, then bake 8–10 minutes more until set. Let the shell cool slightly while preparing filling.

Dark Chocolate Filling

Place the chopped chocolate in a heatproof bowl.

In a small saucepan, heat the heavy cream and granulated sugar over medium heat just until it begins to simmer and steam (do not boil). Remove from heat immediately.

Pour the hot cream over the chopped chocolate and let sit 1–2 minutes to soften the chocolate. Gently whisk until smooth and glossy.

Whisk in the room-temperature butter, vanilla extract, and fine sea salt until fully incorporated and the ganache is silky.

Pour the warm ganache into the pre-baked tart shell, smoothing the top with an offset spatula. Tap the pan gently on the counter to release any air bubbles.

Let the tart cool to room temperature, then refrigerate for at least 2 hours (or up to overnight) until the filling is set.

Whipped Cream & Assembly

About 15–20 minutes before serving, chill a medium bowl and whisk (or beaters) in the freezer for 5–10 minutes to help the cream whip more quickly.

Pour the heavy whipping cream into the chilled bowl, add confectioners' sugar and vanilla extract. Whip by hand or with an electric mixer on medium-high speed until soft peaks form. Taste and adjust sweetness if desired.

Spoon or pipe dollops of whipped cream onto slices of the chilled tart. Lightly dust with grated dark chocolate or cocoa powder and finish with a small pinch of flaky sea salt on each slice if using.

Serve immediately. Store any leftover tart covered in the refrigerator for up to 3 days; whipped cream is best made fresh.