Make the custard: Bring the milk to a simmer in a medium saucepan over medium heat. Whisk the egg yolks, cornstarch, sugar and salt in a large bowl. Gradually pour the hot milk into the egg mixture, whisking constantly. Pour the mixture back into the saucepan and cook, whisking constantly, until thick and bubbling, about 3 minutes. Strain through a fine-mesh sieve into a bowl, pressing the custard through with a rubber spatula. Stir in the butter and vanilla until smooth.
Set aside 1 or 2 muffin slices for crumbling on top. In 6 wide jars or a 2-quart souffle dish, layer the muffin slices, banana slices and custard; crumble the reserved muffin slices on top. Cover and refrigerate 2 to 4 hours before serving.