Make the crust: Whisk together egg yolk and cream in a small bowl until combined; set aside. Process flour, sugar, and salt in a food processor until combined, about 10 seconds. Scatter butter over flour mixture; pulse until butter resembles small peas, 10 to 12 pulses. With food processor running, add reserved egg yolk mixture; process until dough just comes together, about 1 minute. Transfer dough to a sheet of plastic wrap; wrap tightly, and shape into a 6-inch disk. Refrigerate until firm, at least 1 hour or up to 3 days.

Roll dough into a 12-inch circle on a lightly floured work surface, about ⅛-inch thick. Carefully transfer dough to a 10-inch round tart pan with 1-inch-tall sides and a removable bottom; gently press into corners and fluted sides of pan. Run a rolling pin over top of tart pan to remove any excess dough. Reserve excess dough for patching any holes; cover using plastic wrap, and set aside. Gently push up sides of Crust to expose about 1/16-inch. Prick Crust all over using tines of a fork; freeze, uncovered, until firm, at least 30 minutes or up to 24 hours.

Preheat oven to 350°F. Loosely cover Crust using 2 layers of aluminum foil, and fill with pie weights or dried beans. Place on a large rimmed baking sheet; bake in preheated oven until Crust is just set, about 25 minutes. Remove foil and pie weights. Use reserved excess dough to patch any holes, if needed. Bake Crust at 350°F until golden all over and firm, 10 to 15 minutes. Remove from oven, and let Crust cool completely in tart pan on a wire rack, about 20 minutes.

While crust is cooling, make the caramel filling: Heat cream, butter, and salt in a small saucepan over medium-low, stirring occasionally, until butter melts, about 5 minutes. Remove from heat, and set aside. Bring sugar, water, and corn syrup to a boil in a large saucepan over medium, stirring constantly using a fork. Boil over medium, swirling pan often but not stirring, until mixture begins to turn golden and a candy thermometer registers 325°F, about 10 minutes. Turn off heat; whisking constantly, gradually whisk in cream mixture. Bring to a boil over medium; boil, undisturbed, until a candy thermometer registers 235°F to 240°F, about 5 minutes. Pour caramel filling into cooled crust. Let cool completely at room temperature, about 1 hour, 30 minutes.

Make the ganache: Place chocolate in a medium heatproof bowl; set aside. Bring cream to a simmer in a small saucepan over medium, stirring constantly. Pour hot cream over chocolate; let stand, undisturbed, for 5 minutes. Gently stir until chocolate is melted and smooth, about 1 minute. Pour over cooled caramel filling in crust; spread into an even layer, covering caramel filling completely. Refrigerate, uncovered, until fully set, about 1 hour. Sprinkle with flaky sea salt; slice and serve. (If refrigerating tart for longer than 1 hour, let stand at room temperature for 5 to 10 minutes before slicing.)