Instructions:
Prepare for Baking: Preheat your oven to 350°F (175°C). Grease two standard 6-cavity donut pans thoroughly with butter or non-stick spray.
Make the Crumble Topping: In a small bowl, combine the ½ cup flour, ⅓ cup brown sugar, and ¼ teaspoon cinnamon for the topping. Add the cold, cubed butter and use your fingers or a pastry cutter to work it into the dry ingredients until the mixture resembles coarse crumbs. Set aside.
Combine Dry Ingredients: In a large mixing bowl, whisk together the 2 cups of flour, ¾ cup brown sugar, baking powder, baking soda, 1 ½ teaspoons cinnamon, nutmeg, and salt.
Combine Wet Ingredients: In a separate medium bowl, whisk the egg until uniform. Then, whisk in the buttermilk, melted (and cooled) butter, and vanilla extract until fully combined.
Form the Batter: Pour the wet ingredient mixture into the dry ingredients. Use a spatula to gently fold everything together until just combined; be careful not to overmix. A few lumps are fine. Gently fold in the finely diced apple.
Fill Pans and Add Topping: Spoon the batter into a large piping bag or a resealable plastic bag with a corner snipped off. Pipe the batter evenly into the prepared donut pans, filling each cavity about ⅔ full. Generously sprinkle the prepared crumble topping over the batter in each cavity, pressing down lightly so it adheres.
Bake and Cool: Bake for 12-15 minutes, or until the donuts spring back lightly when touched and a toothpick inserted into the cake comes out clean. Allow the donuts to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.
Glaze (Optional): If using the glaze, whisk the powdered sugar, 1 tablespoon of milk or cider, and vanilla together in a small bowl. Add more liquid as needed to reach a drizzling consistency. Drizzle over the cooled donuts.
