Preheat the oven to 350 degrees F and line a baking sheet with a Silpat or parchment paper.
Mix butter, sugar egg yolk and vanilla until well combined.
Stir into flour, salt, ½ teaspoon cinnamon and nutmeg until just combined.
Roll into 1 inch balls.
In a small bowl, combine chopped nuts and remaining cinnamon. Getnly whisk the egg white in a seperate bowl until light and frothy.
Roll each ball in egg white and then the nuts mixture.
Place each ball onto the baking sheet around an inch apart.
Using your thumb or the back of a spoon, create a small hole in the middle of each cookie being careful not to go all the way through (the "thumbprint").
Bake uncovered for 10-12 minutes. Cool completely.
Fill the thumbprints with apple butter. Serve immediately.
Notes
Make sure your butter is softened but not melted, since this helps the dough come together with the right texture.
Don't press the thumbprint all the way through the cookie. A well is needed to properly hold the apple butter filling.
Use the back of a rounded spoon instead of your thumb for a cleaner, more uniform indent if you want a neater presentation. I used my wooden spoon for this.
Optional Upgrade: Toast the nuts lightly in a dry pan before chopping for a deeper, richer flavor in the coating.
Let the cookies cool completely before filling them, so the apple butter doesn't melt into the dough.
This apple cookies recipe is easy to customize for a nut-free version. Just roll the dough in a mix of 1 tablespoon of sugar and 1 teaspoon of cinnamon instead of nuts.
Ingredients to prep ahead- Chop the nuts and measure out the dry ingredients up to a day in advance.
Leftovers and storage- Store covered at room temperature for up to 4 days.