Preheat the oven to 350 degrees F. Line two baking sheets with parchment paper.
Add the vanilla cake mix, applesauce, and finely chopped apple pie filling to a large mixing bowl. Stir with a silicone spatula until no dry cake mix remains and a thick, sticky dough forms.
Using a 1½-tablespoon cookie scoop, portion the dough onto the prepared baking sheets, spacing the cookies about 2 inches apart.
Lightly flatten the tops of the dough portions with the back of the scoop or damp fingertips.
Bake for 7 to 9 minutes, or until cracks appear on top and the edges look set. The cookies will remain pale and should still be soft in the centers.
Let the cookies cool on the baking sheets for 10 minutes. Transfer them to a wire cooling rack and cool completely.
Spread the vanilla frosting over the cooled cookies before serving.
Notes
Finely chop the apple pie filling before measuring it. Smaller apple pieces distribute more evenly and make the dough easier to scoop.
The dough will be soft and sticky. Use a cookie scoop rather than rolling it with your hands.
Watch for cracks instead of browning. These cookies remain pale even when they are fully baked. The original recipe also identifies surface cracks as the main doneness cue.
Do not frost warm cookies. Even slightly warm cookies can cause the frosting to melt and slide off.
For a different flavor, replace the vanilla frosting with prepared cream cheese or caramel frosting.
Store frosted cookies in a single layer in an airtight container in the refrigerator for up to 4 days. Let them sit at room temperature for about 15 minutes before serving.
To make the cookies ahead, bake and cool them up to 1 day before serving. Store them covered at room temperature and add the frosting before serving.
Freeze the unfrosted cookies in an airtight container for up to 2 months.
Place parchment paper between layers. Thaw at room temperature, then frost before serving.