In a small saucepan, combine the apple juice and granulated sugar over medium heat.
Stir occasionally until the sugar dissolves and the mixture begins to simmer.
In a small bowl, mix the cornstarch with cold water until smooth to create a slurry.
Slowly whisk the cornstarch slurry into the simmering apple juice mixture.
Continue to cook, stirring constantly, until the glaze thickens and becomes translucent, about 2 to 3 minutes.
Remove the saucepan from heat and stir in the unsalted butter, ground cinnamon, and vanilla extract until the butter melts and the glaze is smooth.
Allow the glaze to cool slightly before drizzling over your apple cake.