In a small skillet over medium heat, melt 1 tablespoon butter. Add diced apple, brown sugar, and ½ tsp cinnamon. Cook for 5–7 minutes until tender and caramelized. Set aside to cool.
In a mixing bowl, whisk together flour, granulated sugar, baking powder, baking soda, cinnamon, and salt.
In a separate bowl, mix buttermilk, egg, melted butter, and vanilla. Pour the wet ingredients into the dry and stir until just combined.
Gently fold in the cooked apple mixture.
Preheat and lightly grease your waffle iron. Spoon batter into each mold and cook for 3–4 minutes or until golden brown and cooked through. Transfer to a wire rack to cool slightly.
In a small bowl, whisk together powdered sugar, milk, and vanilla until smooth. Adjust thickness as needed.
Once the waffle donuts are warm (not hot), dip the tops into the glaze or drizzle it over with a spoon. Let set before serving.
