Step 1: Make the Chocolate Tart Crust

1. In a large mixing bowl, sift together the flour and cocoa powder.
2. Add the cubed butter and sugar. Use a pastry cutter or your fingers to rub the butter into the dry ingredients until the mixture resembles coarse crumbs.
3. Stir in the egg yolk and cold water. Mix until a dough begins to form.
4. Knead gently just until it holds together. Do not overwork the dough.
5. Flatten the dough into a disc, wrap in plastic wrap, and refrigerate for at least 30 minutes.
6. Preheat your oven to 350°F (175°C).
7. Roll out the dough on a lightly floured surface to fit a 9-inch tart pan.
8. Gently press the dough into the pan and trim excess edges.
9. Prick the bottom with a fork and chill for another 10 minutes.
10. Bake for 15–18 minutes or until firm. Let it cool completely.

Step 2: Make the Salted Caramel Layer

1. In a heavy-bottomed saucepan, heat the sugar over medium heat. Stir constantly as it melts and begins to darken.
2. Once the sugar is fully melted and golden brown, add the butter. Be cautious—it will bubble vigorously.
3. Stir until the butter is fully incorporated.
4. Slowly pour in the heavy cream, again being careful as it bubbles.
5. Stir until smooth and glossy.
6. Add sea salt to taste, then remove from heat and allow the caramel to cool slightly before pouring.
7. Pour the caramel over the cooled tart crust, spreading evenly.
8. Chill the tart in the refrigerator for about 30 minutes until the caramel sets.

Step 3: Prepare the Chocolate Ganache

1. Place the chopped chocolate in a heat-proof bowl.
2. In a saucepan, heat the cream over medium heat until it just begins to simmer.
3. Pour the hot cream over the chocolate and let it sit for 1–2 minutes.
4. Gently stir with a spatula or whisk until the mixture becomes smooth and glossy.
5. Stir in vanilla extract if using.
6. Pour the ganache over the caramel layer, smoothing the top with a spatula.
7. Return the tart to the fridge and let it chill for at least 1–2 hours until the ganache is set.

Step 4: Add Final Touches and Serve

1. Once the ganache is firm, sprinkle with sea salt flakes or your choice of garnish.
2. Use a sharp knife to slice and serve chilled or at room temperature.