Make the crust: Combine the flour, cocoa powder, sugar and salt in a food processor; pulse a few times to combine. Add the butter and pulse until the mixture looks like sand. Add the egg yolk and 4 teaspoons cold water; pulse until the dough comes together and forms a ball, adding up to 2 more teaspoons water if necessary. Press the dough into a 9-inch fluted tart pan in an even layer. Refrigerate at least 30 minutes.
Preheat the oven to 350˚ F. Line the crust with parchment paper and fill with pie weights or dried beans. Bake until the crust looks dry, 13 to 15 minutes. Remove the paper and weights and bake 5 more minutes. Transfer the crust to a rack to cool completely.
Make the filling: Combine the sugar and water in a medium saucepan over medium-high heat; cover and bring to a boil. Cook, covered, until the sugar is dissolved, 4 to 5 minutes. Uncover and cook until the caramel is a dark amber color, 3 to 5 more minutes.
Remove the caramel from the heat and add the heavy cream; whisk until the caramel stops bubbling. Stir in the vanilla and fine salt. Let cool to room temperature.
Combine the chocolate and butter in a heatproof bowl set over a saucepan of barely simmering water (do not let the bowl touch the water). Let melt completely, about 10 minutes, then stir to combine. Remove the bowl from the pan and let cool.
Preheat the oven to 325˚ F. Whisk the egg yolks and whole egg in a large bowl until frothy, about 2 minutes. Slowly whisk in the caramel, then the melted chocolate mixture. Set the tart pan on a large rimmed baking sheet; pour the filling into the crust.
Transfer the baking sheet to the oven. Bake until the tart is set around the edges but the center is still jiggly, 25 to 30 minutes. Transfer to a rack and let cool to room temperature, then refrigerate until fully set, at least 3 hours. Sprinkle with flaky sea salt before serving.
Tools You May Need
Photograph by Yunhee Kim
Tools You May Need
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