In a large bowl, combine flour, cocoa powder, sugar, and salt. Cut in the butter until the mixture resembles coarse crumbs. Stir in ice water, a tablespoon at a time, until the dough comes together. Shape into a disk, wrap in plastic, and refrigerate for at least 30 minutes.

Bake the Crust

Preheat the oven to 350°F (175°C). Roll out the chilled dough on a lightly floured surface and fit into a tart pan. Prick the bottom with a fork. Bake for 15-20 minutes or until firm. Allow to cool.

Filling Preparation

Prepare the Filling

In a saucepan, heat the heavy cream until just simmering. Remove from heat and add the chopped dark chocolate, stirring until melted and smooth. In a separate bowl, whisk together sugar, eggs, and vanilla extract until combined. Gradually add the chocolate mixture to the egg mixture, stirring continuously.

Bake the Tart

Pour the filling into the cooled crust and bake for 25-30 minutes, or until the filling is set but still slightly jiggly in the center. Let it cool at room temperature, then refrigerate for at least 2 hours.

Topping Preparation

Prepare the Whipped Cream

In a mixing bowl, beat the heavy cream, powdered sugar, and vanilla extract until soft peaks form.

Marble the Chocolate

Melt the milk chocolate and drizzle it over the chilled tart. Use a toothpick or skewer to create a marbled effect. Serve with whipped cream on the side or on top of the tart.