Step 1: Make the Chocolate Tart Crust

1. In a food processor or bowl, mix flour, cocoa powder, powdered sugar, and salt.
2. Add cold butter cubes and pulse (or cut in with a fork) until the mixture resembles coarse crumbs.
3. Stir in egg yolk and ice water until the dough just comes together.
4. Shape into a disc, wrap in plastic wrap, and chill for 30 minutes.

Step 2: Blind Bake the Tart Crust

1. Preheat oven to 350°F (175°C).
2. Roll out dough and press it into a 9-inch tart pan with a removable bottom.
3. Prick the bottom with a fork, then line with parchment paper and baking weights (or dried beans).
4. Bake for 12 minutes, remove the weights, and bake for 5-8 more minutes until set.
5. Let cool completely.

Step 3: Make the Caramel Layer

1. In a small saucepan, heat sugar and water over medium heat, swirling (not stirring) until it turns deep amber.
2. Carefully add heavy cream (it will bubble up), whisking constantly.
3. Stir in butter, vanilla, and sea salt, then remove from heat.
4. Pour caramel into the cooled tart crust and let set for 15 minutes.

Step 4: Make the Chocolate Ganache

1. Place chopped chocolate in a heatproof bowl.
2. Heat heavy cream until just simmering, then pour over the chocolate.
3. Let sit for 1 minute, then whisk until smooth.
4. Stir in butter and vanilla extract for extra richness.
5. Pour ganache over the caramel layer and smooth the top.

Step 5: Chill & Serve

1. Refrigerate for 2-3 hours, until the ganache is set.
2. Garnish with flaky sea salt, chopped nuts, or extra caramel drizzle.
3. Slice and serve with whipped cream or vanilla ice cream.