Step 1

Beat butter, sugar, and brown sugar for cookie dough in a bowl with an electric mixer until creamy. Stir in eggs and vanilla.

Step 2

Combine flour, salt, baking soda, cinnamon, and apple pie spice in a separate bowl. Gradually mix into the wet ingredients until incorporated. Mix in oats and walnuts. Cover and chill in the refrigerator for at least 1 hour.

Step 3

Preheat the oven to 375 degrees F (190 degrees C). Set out two 24-cup mini muffin tins. Place paper towels under 2 wire racks.

Step 4

Combine apples, butter, sugar, and cinnamon for filling in a microwave-safe bowl; microwave on high for 3 minutes, stirring halfway through to distribute the butter, until apples have softened. Transfer to a strainer to drain and cool.

Step 5

Roll chilled dough into walnut-sized balls. Fill each muffin cup with a ball and press down to flatten slightly.

Step 6

Bake in the preheated oven for 10 minutes. Remove and cool for 2 minutes. Press a shot glass down in the middle of each cookie to form it into a cup shape. Transfer from the pan to the prepared wire racks to cool.

Step 7

Put apple filling into the center of each cookie, making sure it's evenly distributed to fill all cookies.

Step 8

Beat egg whites for icing in a glass, metal, or ceramic bowl until foamy. Mix in caramel syrup, cinnamon, and cream of tartar. Gradually mix in 2 cups powdered sugar and mix until it reaches a good drizzling consistency, similar to that of the caramel syrup. Add more powdered sugar if necessary; you don't want it to be so thin that it will run.

Step 9

Drizzle icing over the cookies; it's okay if it melts into the apple filling.

Step 10

Enjoy right away while they're super fresh, or wait until icing hardens, about 3 hours.