Heat oven to 350°F. Stir together graham cracker crumbs, melted butter, and sugar until the texture of wet sand. Press firmly into the bottom and up the sides of a 9-inch pie plate using the bottom of a measuring cup. Bake until fragrant and set, 8 to 10 minutes. Cool completely on a wire rack.

In a medium saucepan, bring 2 cups milk to a bare simmer over medium heat. Remove from heat and whisk in chopped chocolate until completely smooth. Set aside.

In a large bowl, vigorously whisk egg yolks, cornstarch, sugar, and remaining 1/2 cup milk until smooth and pale yellow, about 1 minute.

Slowly drizzle half the hot chocolate milk into the yolk mixture, whisking constantly. Pour back into the saucepan and cook over medium heat, whisking constantly, until the mixture thickens and large bubbles burst on the surface, 3 to 4 minutes. It should coat the back of a spoon and hold a line when you run your finger through it.

Remove from heat and whisk in cold butter cubes and vanilla until glossy. Press plastic wrap directly onto the surface to prevent a skin from forming. Refrigerate until cool to the touch, about 45 minutes.

Spread half the cooled custard into the crust. Arrange banana slices in a single layer, pressing them gently into the custard. Top with remaining custard and smooth the surface. Press plastic wrap directly on top and refrigerate at least 3 hours, preferably overnight, until fully set.

Just before serving, beat cold heavy cream and powdered sugar to stiff peaks. Spread or pipe over the pie. Slice with a sharp knife wiped clean between cuts.