Soft, chewy oatmeal cookies bursting with fresh apple, warm cinnamon and spices, then finished with a silky maple icing for a quintessential fall treat.
Place applesauce in a medium saucepan over medium heat; bring to a low boil, then simmer until reduced by half to about 1/3 cup (80g), ~10 minutes. Transfer to a heatproof bowl and cool to room temperature.
Preheat oven to 350°F (177°C). Line large baking sheets with parchment paper or silicone baking mats.
In a large bowl, whisk together oats, flour, baking soda, salt, cinnamon, allspice, and nutmeg.
Using a hand or stand mixer fitted with a paddle, beat the butter, granulated sugar, and brown sugar on medium speed until creamed, about 5 minutes, scraping bowl as needed.
Add the reduced, cooled applesauce, egg yolk, and vanilla; beat on medium-high until combined. Scrape bowl as needed.
Add dry ingredients and beat on low until combined.
Beat in finely diced apples and walnuts (if using). Dough will be thick and sticky.
Using a medium cookie scoop, portion about 1.5 Tbsp (35g) dough mounds and place 3 inches apart on prepared sheets.
Bake 14–16 minutes, until lightly browned and set around the edges; centers will look very soft.
Cool cookies on the baking sheet for 10 minutes, then transfer to a wire rack to cool completely.
For the maple icing: In a small saucepan over low heat, melt butter with maple syrup, whisking occasionally. Remove from heat and whisk in the sifted confectioners’ sugar; add a pinch of salt to taste.
Drizzle icing over cooled cookies and let set for about 1 hour.
Storage: Store in an airtight container at room temperature up to 2 days or refrigerate up to 1 week. Dough and baked cookies freeze well (see notes in source).