Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper or a silicone baking mat and set aside.

In a large mixing bowl, using a hand mixer or a stand mixer fitted with the paddle attachment, cream together the butter, brown sugar, and granulated sugar until light and fluffy, about 2-3 minutes.

Scrape down the sides and bottom of the bowl with a rubber spatula. Add the egg and beat on medium-low speed for 30 seconds, until fully incorporated and smooth.

In a separate bowl, whisk together the flour, cornstarch, rolled oats, cinnamon, nutmeg, baking powder, and salt.

Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Be careful not to overmix. Scrape down the sides and bottom to ensure everything is evenly incorporated.

Use a 3-tablespoon-sized cookie scoop to portion out the dough. Press two scoops of dough together to form larger cookies. This will make seven large cookies. Roll each into a ball in the palm of your hand.

Place 3-4 dough balls on the prepared baking sheet, spacing them about 3 inches apart.

Bake in the preheated oven for 14-15 minutes, or until the edges are golden brown but the centers are still slightly soft. The cookies will continue to set as they cool.

Remove from the oven and, while still warm, gently press the cookies down to about ½ inch thick using a flat spatula or the bottom of a cup.

Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.

In a large bowl, combine the diced apples, lemon juice, sugar, cinnamon, and a pinch of salt. Toss until evenly coated.

Place the apple mixture in a medium-large pot over medium-high heat. Stir occasionally as the apples cook and release their juices, bringing the mixture to a slow boil.

In a small bowl, mix the cornstarch with cold water to create a slurry.

Once the apples have slightly softened, add the slurry to the pot, stirring constantly. The mixture should thicken within 30 seconds. If it doesn’t, you may need to add a bit more slurry.

Remove from heat and transfer the apple mixture to a container. Allow it to cool completely before using. The filling can be stored in the refrigerator in an airtight container for up to 1 week.

Preheat your oven to 400°F (205°C). Line a baking sheet with parchment paper and set aside.

In a medium bowl, combine the brown sugar, flour, oats, butter, cinnamon, and salt. Mix with a wooden spoon until the mixture forms clumps resembling sand.

Spread the streusel mixture evenly on the prepared baking sheet.

Bake for 8-10 minutes, stirring halfway through. The streusel should be golden brown. Allow it to cool completely, then crumble into smaller pieces if necessary.

In the bowl of an electric mixer, combine the softened butter and cream cheese. Cream on medium speed for about 2 minutes. Scrape down the sides and bottom of the bowl, then beat for another 2 minutes.

With the mixer on low speed, gradually add the powdered sugar, followed by the vanilla, ground cinnamon, and salt. Once combined, increase the speed to medium-high and beat for an additional 2-3 minutes until smooth and fluffy.

Once the cookies have cooled completely, begin assembling.

Place the cinnamon cream cheese frosting in a piping bag or a ziplock bag with a medium-sized hole cut in one corner. Set aside.

Take each cookie and top with 1-2 tablespoons of the apple filling, spreading it evenly across the top.

Pipe a swirl of frosting on top of the apple filling.

Sprinkle the cinnamon streusel over the frosting. Serve and enjoy