FOR CRUST
28 chocolate wafers such as Nabisco Famous, finely ground in a food processor (1 1/2 cups)
3/4 stick (6 tablespoons) unsalted butter, melted and cooled completely

FOR FILLING
1/2 lb fine-quality bittersweet chocolate (no more than 60% cacao if marked), coarsely chopped
3/4 stick stick (6 tablespoons) unsalted butter, cut into 1/2-inch cubes
2 large eggs, lightly beaten
1/3 cup heavy cream
1/4 cup granulated sugar
1/4 teaspoon salt
1 teaspoon pure vanilla extract

SPECIAL EQUIPMENT: an 8-inch (20-cm) round springform pan

GARNISH: unsweetened cocoa powder for sprinkling