Preheat oven to 350°F.
Yield: 8" tart
Melt butter for crust; set aside to cool.
Wrap a sheet of foil over the outside bottom of an 8" springform pan, in case of any leakage. Lightly butter
the inside of the pan.
Stir together the ground wafers and melted butter. Pat this mixture evenly onto the bottom of the pan and up about 1 1/2" of the sides. Bake until the crust is slightly puffed, about 10 minutes. Cool completely on a rack, for about 15 minutes. (Leave the oven on.)
Melt the chocolate and butter in a 2-quart heavy saucepan or in the microwave, just until melted. Remove from the heat and cool for 5 minutes.
Whisk eggs, cream, sugar, salt, and vanilla. Whisk chocolate mixture into the egg mixture until well combined.
Pour the filling into the cooled crust and rap the pan once on the counter to eliminate any air bubbles. Bake until the filling 1" from the edge is set and slightly puffed but the center still trembles slightly when the pan is gently shaken. This takes about 20-25 minutes. The center will continue to set as it cools.
Cool the tart completely in the pan, on a rack, for about 2 hours. Chill, uncovered, until the center is firm, about 4 hours. Remove the sides from the pan and sprinkle with unsweetened cocoa to serve.
This tart can be chilled for about 3 days. Cover loosely after the tart is completely chilled. The crust, without the filling, can be made a day in advance and kept covered at room temperature.