Melt the butter in a skillet over medium heat.
Add the diced Granny Smith apples and cook for about 2 minutes, just to soften them slightly.
Stir in the brown sugar, cinnamon, and vanilla. Cook until the apples are tender and glossy.
In a small bowl, whisk together the cornstarch and water to make a slurry.
Add the slurry and heavy whipping cream to the apples.
Cook for another few minutes, stirring, until the mixture thickens slightly.
Remove from the heat and let the apple pie filling cool completely before using it in recipes.
