Preheat oven to 350°. Line 2 baking sheets with parchment paper.
In a large bowl, beat butter and brown sugar with a mixer at medium speed until creamy, 3 to 4 minutes, stopping to scrape sides of bowl. Beat in egg and buttermilk.
In a medium bowl, whisk together flour, baking soda, and salt. Reduce mixer speed to low. Gradually add flour mixture to butter mixture, beating just until combined. Stir in oats, white chocolate morsels, and macadamia nuts. Using a 2-tablespoon cookie scoop, scoop dough 3 inches apart onto prepared pans. Press batter to ¼-inch thickness.
Bake until lightly browned, 12 to 14 minutes. Let cool on pans for 2 minutes. Remove from pans, and let cool completely on wire racks.
Recipe by Southern Lady Magazine at https://southernladymagazine.com/white-chocolate-macadamia-peach-ice-cream-sandwich-recipe/