In heatproof bowl set over simmering water, melt unsweetened chocolate. In small cup, sprinkle gelatin over 2 tablespoons cold water; let stand. In saucepan, whisk together egg yolks, 1/4 cup of the sugar and the salt. Gradually whisk in milk. Cook over low heat, stirring constantly, until mixture coats a spoon (180°F on a candy thermometer). Remove from heat; stir in gelatin until dissolved, then whisk in chocolate and vanilla. Set saucepan in a bowl of ice water; stir constantly with a spatula until cold, making sure to scrape the bottom of the saucepan. Remove from ice water; transfer chocolate mixture into a large mixing bowl. In bowl with electric mixer, beat egg whites until foamy; gradually beat in remaining 1/4 cup sugar. Continue beating to soft peaks; do not overbeat. Fold egg whites into chocolate mixture; turn mixture into pie crust. Refrigerate at least 4 hours or overnight.
In saucepan, melt butter and brown sugar. When mixture is bubbling, whisk in 2 tablespoons water until smooth. Add bananas; cook about 1 minute or until sauce thickens and coats bananas. Cool, spoon over top of pie. (Bananas can also be served warm; top each slice with a spoonful of bananas just before serving.)
To make pie crust from scratch, heat oven to 350°F. In food processor, process 1 1/2 cups crushed chocolate wafers to coarse crumbs. Add 4 tablespoons melted butter; process to fine crumbs. Press evenly into the bottom of a 9-inch pie dish. Place on baking sheet; bake 10 minutes. Cool.
To cut pie, run a sharp knife under hot water between cutting slices.