In a 1 1/2- to 2-quart pan, combine 3/4 cup sugar, cornstarch, and salt. Add milk and whisk to blend. Whisk over medium heat until mixture boils and thickens, taking care not to scorch. Remove from heat.
In a small bowl, beat egg yolks to blend. Stir about 1/2 cup of the hot milk mixture into yolks, then stir mixture back into pan. Return pan to medium-low heat and stir until mixture just begins to bubble, about 2 minutes. Remove from heat and stir in butter and 1 teaspoon vanilla.
Peel bananas. Slice half into chocolate pastry, covering bottom evenly. Pour half the warm custard evenly over bananas. Repeat to layer remaining bananas and custard. Cover surface of custard with plastic wrap. Chill until cold, at least 2 hours.
In a bowl, with a mixer on high speed, beat whipping cream and remaining 2 tablespoons sugar and 1 teaspoon vanilla until mixture holds soft peaks. Mound on pie and swirl to cover decoratively. Top with chocolate curls, if desired.
Chocolate pastry: In a food processor or a bowl, combine 1 cup all-purpose flour, 1/4 cup sugar, 3 tablespoons unsweetened cocoa, and 1/4 teaspoon salt; whirl or mix to blend. Add 1/2 cup (1/4 lb.) butter or margarine, cut into chunks; whirl or rub in with your fingers until fine crumbs form. Add 1 large egg yolk and 1/2 teaspoon vanilla; whirl or mix with a fork until blended. Pour mixture into a 9-inch pie pan and press evenly over bottom and up sides to edge of rim. Bake in a 350° regular or convection oven just until crust is slightly darker around edges, 18 to 20 minutes. Let cool on a rack at least 1 hour.