Chocolate Truffle Tart

INGREDIENTS

• 3/4 stick (6 tablespoons) unsalted butter, cut into 1/2-inch cubes
• Garnish: unsweetened cocoa powder for sprinkling
• 1/4 teaspoon salt
• 1/2 lb fine-quality bittersweet chocolate (no more than 60% cacao if marked), coarsely chopped
• 3/4 stick (6 tablespoons) unsalted butter, melted and cooled completely
• 28 chocolate wafers such as Nabisco Famous, finely ground in a food processor (1 1/2 cups)
• 1/2 cup heavy cream
• 2 large eggs, lightly beaten
• 1 teaspoon pure vanilla extract
• 1/4 cup granulated sugar