1. To make the crust, preheat the oven to 375°F (190°C). Mix the graham cracker crumbs, butter, and sugar until combined. Press firmly and evenly in a 9in (23cm) tart pan with a removable bottom. Bake for about 10 minutes, or until the crust looks set. Let cool completely on a wire rack.

2. To make the filling, sprinkle the gelatin over ¼ cup water and let stand about 5 minutes, until spongy. Meanwhile, heat the cream in a medium saucepan over medium heat until simmering. Remove from the heat, add the chocolate and vanilla, and stir until the chocolate melts. Add the softened gelatin and whisk well until dissolved.

3. Meanwhile, using an electric mixer, beat the 3 egg yolks with ½ cup of the sugar and the salt for about 2 minutes, until the mixture is thick and creamy. Slowly beat in the chocolate mixture, mixing until well blended. Cover the bowl with plastic wrap and refrigerate about 15 minutes, or until cool and on the verge of setting.

4. Beat the egg whites and cream of tartar in a large bowl until soft peaks form. Beat in the remaining ¼ cup sugar, one tablespoon at a time, until stiff peaks form. Remove the chocolate mixture from the refrigerator and beat with the mixer for 2 minutes. Stir in about one-fourth of the whites, then fold in the remainder. Spoon the chocolate mixture into the crumb crust and smooth the top.

5. In a separate bowl, beat the cream with the confectioner's sugar until stiff peaks form. Swirl the whipped cream over the filling. Grate additional chocolate over the cream. Refrigerate for at least 2 hours. Remove the sides of the pan and serve chilled.