1. Make the crust: Stir together graham cracker crumbs, butter, sugar, and salt in a bowl. Press into bottom and up sides of a 9-inch pie plate. Refrigerate until firm, about 30 minutes. Preheat oven to 350 degrees. Bake until crust is set and begins to turn golden brown, about 12 minutes. Let cool completely on a wire rack.

2. Make the filling: Sift together sugar, cocoa powder, cornstarch, and salt into a medium saucepan. Gradually whisk in milk. Cook over medium-high heat, stirring constantly, until almost boiling. Reduce heat to medium; add chocolate. Cook, stirring constantly, until chocolate has melted and mixture is thick, about 2 minutes. Remove from heat; whisk in butter and vanilla until smooth. Spread chocolate mixture over crust. Refrigerate until cold and firm, about 1 hour.

3. Make the topping: Prepare an ice-water bath; set aside. Sprinkle gelatin over 2 tablespoons cold water in a small bowl. Let stand until soft, about 3 minutes. Whisk 1/4 cup sugar, the cornstarch, and salt in a medium saucepan. Gradually whisk in the milk. Cook over medium-high heat, stirring constantly, until mixture is thick and boiling, about 5 minutes. Remove from heat; stir in gelatin mixture, and let cool completely. Stir in vanilla and rum. Briefly place pan in an ice-water bath to thicken slightly, if needed. Do not let it set completely. Set aside.

4. Put egg whites in the bowl of an electric mixer fitted with the whisk attachment. Beat on medium-low speed until soft peaks form. Reduce speed to low; continue beating while you make syrup.

5. Bring remaining 1/2 cup sugar and 3 tablespoons water to a boil in a saucepan, stirring to dissolve sugar. Brush down sides of pan with a wet pastry brush to prevent crystals from forming. Cook, without stirring, until syrup registers 240 degrees on a candy thermometer.

6. Raise mixer speed to high, and beat egg whites just until stiff peaks begin to form. Immediately pour syrup into egg whites in a slow, steady stream near side of bowl, avoiding whisk. Beat until glossy and cooled, about 7 minutes.

7. Fold meringue mixture into reserved gelatin mixture in 3 batches until just combined. Mound the mixture into the chocolate-filled crust. Refrigerate pie for 2 1/2 hours or overnight before serving.