Bring the juice to a boil over medium heat in a medium saucepan.
While it’s heating, combine the flour, sugar, and salt in a small mixing bowl, and gradually add the egg yolk and the milk.
Add the boiling juice and stir until thickened.
Remove the mixture from the heat.
Beat the egg white with a whisk or hand mixer in a medium mixing bowl until stiff peaks form. Fold the beaten egg white into the elderberry mixture until evenly combined.
Pour the filling into the pie shell.
Bake for 20 to 30 minutes, until the center is bubbly.
Recipe by Amish 365 at https://www.amish365.com/summer-amish-specialty-elderberry-jam-elderberry-custard-pie/