Crust: Whisk together flour, cocoa powder, and salt until combined. Beat butter with confectioners' sugar on medium speed until pale and fluffy, about 2 minutes. Add egg yolk and beat until combined. Add flour mixture and beat on low speed until just combined and crumbly (do not overmix). Pat dough into a disk; wrap in plastic. Refrigerate at least 1 hour and up to 2 days, or freeze up to 1 month.

On a lightly floured piece of parchment, roll dough into a 12-inch circle. Fit into a 9-inch tart pan with a removable bottom. With a paring knife, trim dough flush with rim; refrigerate until firm, 30 minutes.

Blind-bake crust: Preheat oven to 375 degrees. Prick bottom of tart all over with a fork. Bake until set and dough is dry, 18 to 20 minutes. Let cool completely.

Filling: Combine granulated sugar, corn syrup, and 1/4 cup water in a small saucepan over medium-high heat and cook, stirring gently, until sugar is dissolved. Continue to cook, without stirring, until syrup comes to a boil, occasionally washing down sides of pan with a wet pastry brush to prevent crystals from forming. Cook until syrup is amber in color, 8 to 10 minutes. Remove from heat. Carefully add heavy cream (caramel will steam and spatter). Add butter, vanilla, salt, and chocolate; stir until melted and smooth. Add pecans; stir until combined. Transfer mixture to tart crust. Let stand until set and cool, about 2 hours. Tart can be served room-temperature or refrigerated, uncovered, up to 3 days. Let stand at room temperature 30 minutes and sprinkle with flaky salt before serving.