Make the pastry dough: In a mixing bowl, combine flour and powdered sugar. Rub in the cold butter with your fingers until the mixture resembles breadcrumbs. Add egg yolk and 1 tablespoon of cold water. Mix until a dough forms. Add a second tablespoon of water only if needed.

Chill the dough: Flatten into a disk, wrap in plastic wrap, and refrigerate for at least 20–30 minutes to rest and firm up.

Roll out and line the tin: On a lightly floured surface, roll out the dough to fit a 9-inch tart tin. Press gently into the edges and trim the excess. Prick the base with a fork.

Blind bake the crust: Preheat the oven to 375°F (190°C). Line the tart shell with parchment paper and fill with baking beans or rice. Bake for 12 minutes. Remove paper and weights, then bake for another 5–8 minutes until lightly golden. Cool completely.

Heat the cream: In a saucepan, bring the heavy cream just to a simmer over medium heat. Do not let it boil.

Melt the chocolate: Place the chopped chocolate in a heatproof bowl. Pour the hot cream over it. Let it sit for 1 minute, then stir slowly until it is smooth and fully melted.

Add butter and salt: Stir in the butter, a pinch of salt, and sugar (if using). Mix until glossy and well incorporated.

Pour and set: Pour the ganache into the cooled tart shell. Tap the tart tin gently to remove any air bubbles and level the top.

Chill to set: Refrigerate for at least 2 hours, or until the filling is firm to the touch.

Decorate and serve: Dust with cocoa powder or top with fresh berries before slicing and serving.