Heat the oven to 350°F and arrange a rack in the lower third. Lightly flour a work surface and roll the dough into a round approximately 12 inches in diameter and 1/4 inch thick. Line a 9-inch pie plate with the dough and fold the excess under itself, crimping the edges to seal. Pierce the base of the crust with a fork several times, line with a piece of parchment paper, and fill with pie weights or dried beans.
Bake until the dough starts to set, about 20 minutes. Remove the pie weights and parchment paper and continue to bake until the dough is dry and slightly brown, about 25 minutes more. Remove from the oven and let cool completely.
For the filling:
Cut bananas into 1/4-inch-thick slices and overlap to fill the bottom of the cooled crust; set aside.
Place cornstarch in a medium bowl and whisk in 1/2 cup of the milk, then the yolks, until incorporated and smooth.
In a large saucepan whisk together coconut milk, sugar, salt, and remaining milk and bring to a boil over medium heat. Once boiling, slowly whisk in yolk mixture and reduce heat to medium low. Using a rubber spatula, constantly stir and scrape the bottom of the pot to prevent scorching; cook until mixture reaches 180°F on an instant-read thermometer, about 5 to 10 minutes.
Remove from heat and stir in vanilla. Pour hot custard into pie crust and let cool to room temperature, about 15 minutes. Once cool, cover and refrigerate until firm and chilled, at least 30 minutes or up to 12 hours.
For the topping:
When ready to serve, combine cream and powdered sugar in a chilled mixing bowl and whip until medium peaks form. Spread evenly over the surface of the pie and sprinkle toasted coconut over cream. Serve immediately or chill up to 8 hours.