Prepare the crust. For almond flour: Mix almond flour, melted butter, sweetener, and a pinch of salt. Press into a 9-inch pie dish. Bake at 350°F (175°C) for 10–12 minutes, until lightly golden. Cool completely. For a cookie crust, mix crumbs with melted butter, press into dish, and chill to set.

Slice the bananas. Cut 2 bananas into thin coins. Set aside. Reserve the third banana for topping right before serving to avoid browning.

Heat the milk and cream. In a medium saucepan, whisk almond or dairy milk with heavy cream over medium heat until steaming but not boiling.

Make the custard base. In a bowl, whisk sweetener, cornstarch, salt, and egg yolks until smooth. Slowly ladle in 1/2 cup of the hot milk mixture, whisking constantly to temper the eggs.

Thicken the filling. Pour the tempered yolk mixture back into the saucepan. Cook over medium heat, whisking constantly, until thick and bubbling, 3–5 minutes. It should coat the back of a spoon.

Add butter and vanilla. Remove from heat. Whisk in butter and vanilla until glossy. Let it cool for 5 minutes, whisking occasionally to release steam.

Layer the pie. Arrange a single layer of banana slices over the cooled crust. Pour half the custard over the bananas and smooth it out. Add another layer of banana slices, then top with the remaining custard. Gently tap the pie to settle air bubbles.

Chill to set. Press a piece of plastic wrap directly onto the surface of the custard to prevent a skin. Chill for at least 4 hours, preferably overnight, until firm.

Whip the topping. Beat the heavy cream (or chilled coconut cream) with powdered sweetener and vanilla to soft peaks. Spread or pipe over the chilled pie.

Garnish and serve. Slice the remaining banana just before serving and arrange on top. Finish with chocolate shavings or toasted coconut if you like. Slice with a sharp knife and serve cold.