Make the Tart Crust:

1. Prepare dough: In a food processor, combine flour, powdered sugar, and salt. Pulse to mix. Add cold cubed butter and pulse 8-10 times until mixture resembles coarse crumbs with pea-sized butter pieces. Add egg yolk and vanilla. Pulse until dough just starts to come together. If too dry, add ice water 1 tablespoon at a time.
2. Chill dough: Turn dough out onto a piece of plastic wrap, form into a disk, wrap tightly, and refrigerate for at least 30 minutes (up to 2 days). This rest period is crucial for tender pastry.
3. Roll out: On a lightly floured surface, roll chilled dough into an 11-inch circle about 1/8-inch thick. Roll from center outward, rotating dough frequently to maintain round shape. If dough cracks, let it warm up slightly.
4. Line pan: Carefully transfer dough to a 9-inch tart pan with removable bottom. Gently press dough into bottom and up the sides, allowing excess to hang over edges. Trim excess dough with a knife or roll your rolling pin across the top. Prick bottom all over with a fork. Freeze for 15 minutes.
5. Blind bake: Preheat oven to 375°F (190°C). Line tart shell with parchment paper and fill with pie weights or dried beans. Bake for 15 minutes. Remove weights and parchment, and bake another 8-10 minutes until golden brown. Cool completely on a wire rack.

Make the Chocolate Filling:

6. Chop chocolate: While crust cools, finely chop chocolate and place in a heatproof bowl. The finer you chop, the smoother your ganache will be.
7. Heat cream: In a medium saucepan, heat heavy cream over medium heat until it just begins to simmer (small bubbles around the edges). Don’t let it boil! If using espresso powder, whisk it into the hot cream.
8. Make ganache: Pour hot cream over chopped chocolate. Let sit undisturbed for 2 minutes to melt the chocolate. Then whisk gently from the center outward until completely smooth and glossy. Add butter, vanilla, and salt. Whisk until butter melts and mixture is silky smooth.
9. Fill tart: Pour chocolate ganache into the cooled tart shell. Gently tap the pan on the counter to release any air bubbles. Use an offset spatula to smooth the top if needed.
10. Chill: Refrigerate tart for at least 3 hours or overnight until filling is set. The ganache should be firm but not hard.
11. Serve: Remove outer ring from tart pan. Slice with a hot, dry knife (wipe clean between cuts) and serve at room temperature for best flavor and texture. Garnish with whipped cream, berries, or sea salt flakes if desired.