- Crust: Pulse flour with cocoa, sugar, coconut and salt in a food processor. Pulse in butter until finely crumbed. Whisk egg yolk with 2 tbsp (30 mL) ice water; pulse into flour mixture until it clumps together.
- Press dough evenly into the bottom and 1 1/2 inches (4 cm) up the sides of a 10-inch (25 cm) springform pan. Prick with a fork. Freeze for 15 minutes. Preheat oven to 375°F (190°C). Bake, on a lower rack, for 15 minutes or until set. Cool to room temperature.
- Filling: Whisk milk with sugar and cornstarch in a heavy saucepan set over medium heat. Bring to a boil, stirring often, until thickened. Whisk some of the hot milk mixture into egg yolks. Whisk back into remaining milk. Cook, whisking, for 1 minute. Remove from heat; stir in butter and vanilla.
- Line cooled shell with bananas. Pour hot filling over top to cover bananas completely. Cool to room temperature; chill for at least 2 hours or up to 2 days. Just before serving, remove ring and arrange on serving platter. Pipe coconut whipped cream to cover custard completely and garnish with toasted coconut.
- To make individual pies: Divide dough between eight, 5-inch (12-cm) mini pie plates or foil pot-pie pans; pressing evenly into bottom and up sides. Arrange on a baking sheet; bake for 15 minutes. Cool to room temperature. Divide bananas and custard between the cooled shells. Chill until set. Garnish with whipped cream and toasted coconut.