Preheat your oven to 350°F (180°C) and line baking sheets with parchment paper. This ensures easy cleanup and prevents the cookies from sticking.
In a stand mixer fitted with a paddle attachment, beat 1 cup of unsalted butter and both sugars (¾ cup granulated sugar and ¾ cup light brown sugar) at medium speed until the mixture is fluffy, about 3 to 4 minutes. Make sure to stop and scrape the sides of the bowl to incorporate all the ingredients well.
Add 2 large eggs, one at a time, mixing well after each addition for a uniform texture. Then, beat in 1 teaspoon of vanilla extract until combined.
In a separate medium bowl, sift together 2¼ cups of all-purpose flour, 1 teaspoon of kosher salt, ½ teaspoon of baking powder, ½ teaspoon of baking soda, ½ teaspoon of ground cinnamon, and ½ teaspoon of apple pie spice. This step not only mixes the dry ingredients but also aerates the flour, ensuring a lighter cookie.
With the mixer on low speed, gradually add the flour mixture to the butter mixture. Mix until just combined, stopping to scrape the sides of the bowl to ensure everything is well mixed.
Add 2 cups of quick-cooking oats, ½ cup of sliced almonds, and both the chopped candied cherries and dried cherries. Beat until fully integrated, making sure to scrape the bowl again.
Using a 1½-tablespoon spring-loaded scoop, portion out the dough, about 32 grams each, and place them 2 inches apart on the prepared baking sheets. This spacing allows the cookies to spread during baking without merging.
Bake the cookies until the edges are golden brown and the tops are lightly browned, about 10 to 15 minutes. Keep an eye on them as oven temperatures can vary.
After baking, let them cool on the pans for 10 minutes before transferring them to wire racks to cool completely. This rest period helps them set up nicely.
Prepare the Cherry Buttercream by spooning it into a pastry bag fitted with a small French star piping tip (Ateco #863). Pipe buttercream onto the flat side of half the cookies, then sandwich with the remaining cookies, flat side down. These will be your delicious Fruitcake Cookie Sandwiches.
Store the finished sandwiches in an airtight container for up to 4 days, though I doubt they’ll last that long!