For the Crust

1. In a stand mixer fitted with a paddle attachment, cream the butter, sugar, and salt on medium speed until pale and creamy, about 2 minutes. Scrape down the sides and bottom of the bowl with a rubber spatula. Add the flour and mix on low speed for about 30 seconds, until the flour is incorporated. The mixture will look like wet, clumpy sand. Add the egg yolk and mix on low speed until the yolk is evenly incorporated and the dough is clumpy, about 30 seconds. Using your hand, lightly knead the dough into a ball inside the bowl. Remove the dough from the bowl, press it into a 6-inch disk, wrap it tightly in plastic wrap, and let it rest in the refrigerator for about 30 minutes.
2. Lightly spray a 9.5-inch tart pan with a removable bottom with nonstick cooking spray. Pinch off pieces of dough and press them against the sides of the pan to reach between ⅛ and ¼ inch thick. Using the heel of your hand, press the rest of the dough evenly into the bottom of the pan. (It will look like a mess, and it may initially seem like you won't have enough dough, but have faith, it will come together. If you run out of dough, you’ve made the crust too thick.) Be sure there are no seams in the dough and press it squarely along the corners where the bottom meets the sides of the pan to avoid extra-thick edges. Use a paring knife to trim the top edge of the dough so it is even with the rim of the pan. Press any scraps into the bottom crust. Cover with plastic wrap and place in the freezer for at least 30 minutes.
3. Preheat the oven to 350°F and set an oven rack in the middle position.
4. Place the chilled crust on a baking sheet (for easy handling) and bake until dry and very lightly golden, about 20 minutes. Let cool on the baking sheet on a wire rack while you prepare the filling.

For the Chocolate Filling

1. Place the chocolate in a medium bowl. In a small saucepan, bring the heavy cream to a boil. Pour over the chocolate and let sit for a few minutes, then whisk until melted and smooth. Add the sugar and salt and whisk until well incorporated. Add the eggs and vanilla and whisk until completely smooth. Pour the filling into the baked tart shell and pop any air bubbles with a toothpick. Bake for 20 to 25 minutes, until the filling is mostly set but still a bit wobbly in the center. Let cool completely in the pan on a wire rack, about 1 hour. The tart can be refrigerated at this point for up to 2 days (but bring to room temperature before glazing).

For the Chocolate Glaze

1. On the day of serving, pour the cream and corn syrup into a medium microwave-safe bowl. Microwave on high until boiling, 30 seconds to 1 minute. Add the chocolate and let stand for a few minutes to allow the chocolate to soften. Whisk gently until the chocolate is melted and smooth, then whisk in the hot water. Working quickly so it doesn’t cool and thicken, pour the glaze onto the center of the tart, then carefully tilt the tart around to allow the glaze to run evenly to the edges. Let stand until the glaze is set, about 1 hour, and leave out at room temperature until serving.
2. Remove the outer ring from the tart pan. Insert a metal spatula between the crust and pan bottom to loosen the tart and slide it onto a serving platter (or just leave the bottom in place). Cut into wedges, wiping the knife clean between slices, and serve plain or with sweetened whipped cream. Leftovers will keep nicely in a covered container in the refrigerator for up to 4 days (although the glaze will lose its sheen).
3. Make-Ahead Instructions: The tart can be made, without the glaze up to 2 days ahead of time, and refrigerated. Bring to room temperature before glazing.