Make the Tart Crust: In a large mixing bowl, whisk together the flour, cocoa powder, and powdered sugar. Cut in the cold cubed butter with a pastry cutter or your fingers until the mixture resembles coarse crumbs.
Combine with Egg Yolk: Add the egg yolk and 1 tablespoon of iced water, mixing until the dough just comes together (add more water if needed).
Chill the Dough: Shape the dough into a disk, wrap it in plastic wrap, and refrigerate for at least 30 minutes.
Preheat Oven: Preheat your oven to 350°F (175°C).
Blind Bake the Crust: Roll out the chilled dough on a floured surface and press it into a tart pan. Prick the bottom with a fork. Line the crust with parchment paper and fill with pie weights or dried beans. Bake for 15 minutes, then remove the weights and bake for another 10 minutes until set. Allow to cool.
Prepare the Caramel: In a medium saucepan over medium heat, combine the granulated sugar and ¼ cup of water. Stir until the sugar dissolves, then stop stirring and let it boil until it turns a deep amber color, about 5-7 minutes.
Add Cream and Butter: Remove from heat and carefully add the heavy cream (it will bubble), then whisk in the room temperature butter and corn syrup until smooth. Stir in ½ teaspoon of sea salt.
Assemble the Tart: Pour the caramel into the cooled tart shell and let it set in the fridge for at least 30 minutes.
Make the Chocolate Ganache: Heat the remaining heavy cream in a saucepan until just simmering. Pour over the chocolate chips in a bowl and let sit for 2-3 minutes, then stir until smooth.
Combine Layers: Once the caramel is set, pour the chocolate ganache over the top evenly and let it set in the fridge for another hour.
Garnish and Serve: Before serving, sprinkle flaky sea salt on top for an extra touch and an indulgent flavor.
Notes
Chill the tart for best results and serve with a sprinkle of flaky sea salt.