Make the crust: line a nine inch square baking dish with aluminum foil, leaving a 2 inch overhang on two sides. Put 1 stick butter, granulated sugar and cocoa powder in a heat proof bowl and set over a saucepan of simmering water, do not let the bowl touch the water. Cook, stirring, until the butter melts and sugar dissolves. Add the egg and continue to cook, whisking, until the mixture is thick like hot fudge sauce, three to five minutes. Remove from heat and stir in the graham cracker crumbs. Spread in the prepared baking dish. Cover with plastic wrap and refrigerate until set, about 45 minutes.

Make the filling: Beat the cream cheese, 4 tablespoons butter, the vanilla and salt in a bowl with a mixer on medium high speed until creamy. Add confectioners sugar and beat until stiff, then beat in heavy cream until fluffy. Spread over the crust, cover and refrigerate until firm, at least 30 minutes.

Make the topping: Put the chocolate and the remaining 2 tablespoons butter in a microwave-safe bowl. Microwave in 30 second intervals, stirring, until melted. Spread the chocolate over the filling. refrigerate until set, 30 minutes. Remove from the pan and cut into squares. Store in refrigerator for up to one week.