Stir together graham cracker crumbs, melted butter, and sugar until evenly moistened. Press firmly into the bottom and up the sides of a 9-inch pie plate using the bottom of a measuring cup. Freeze for 15 minutes while you make the filling.

In a medium saucepan, whisk together 2 cups milk, sugar, and cornstarch. In a separate bowl, whisk egg yolks with remaining 1/2 cup milk. Heat the milk mixture over medium heat, whisking constantly, until it begins to steam and thicken slightly, about 5 minutes.

Slowly whisk 1/2 cup of the hot milk mixture into the egg yolks to temper, then pour the yolk mixture back into the saucepan. Cook over medium heat, whisking constantly, until the custard bubbles and thickens to pudding consistency, 2-3 minutes. It should coat the back of a spoon.

Remove from heat and immediately whisk in butter and vanilla until smooth. Press plastic wrap directly onto the surface of the custard to prevent a skin from forming. Refrigerate until cooled to room temperature, about 30 minutes.

Slice bananas into 1/4-inch rounds. Spread half the cooled custard into the crust. Arrange a single layer of banana slices over the custard. Top with remaining custard, spreading to the edges. Press plastic wrap directly on surface and refrigerate at least 2 hours or up to 24 hours.

Just before serving, beat heavy cream and powdered sugar with an electric mixer until soft peaks form. Spread or pipe over the pie. Garnish with additional banana slices if desired.