Prepare the crust:

• In a food processor or mixing bowl, combine flour, powdered sugar, and salt.
• Add cold butter and pulse or cut in with a pastry cutter until the mixture resembles coarse crumbs.
• Add egg yolk and pulse again. If the dough is too dry, add ice-cold water, 1 tablespoon at a time, until it just comes together.
• Form the dough into a disc, wrap in plastic wrap, and chill for 30 minutes.

Bake the crust:

• Preheat oven to 350°F (175°C).
• Roll out the dough on a lightly floured surface and fit it into a 9-inch tart pan with removable bottom.
• Trim excess dough, prick the bottom with a fork, and chill for 10 minutes.
• Line with parchment and fill with pie weights or dried beans.
• Bake for 15 minutes, remove weights, and bake for another 10 minutes until golden. Cool completely.

Make the chocolate ganache filling:

• In a small saucepan, heat the heavy cream until it just begins to simmer (don’t boil).
• Remove from heat and pour over the chopped chocolate. Let sit for 1-2 minutes, then stir gently until smooth.
• Stir in butter, vanilla, and a pinch of salt until fully combined.

Assemble the tart:

• Pour the chocolate filling into the cooled tart shell. Smooth the top with a spatula.
• Refrigerate for at least 2 hours, or until set.

Garnish and serve:

• Before serving, sprinkle with flaky sea salt, fresh berries, or shaved chocolate if desired.
• Serve chilled or at room temperature.