In a large bowl, whisk together flour, brown sugar, baking powder, cinnamon, and salt.
In a separate bowl, mix milk, egg, melted butter or oil, and vanilla extract if using.
Combine wet ingredients into dry ingredients until just blended, then fold in the diced apples.
Preheat the mini waffle maker or waffle iron and lightly grease it with oil or nonstick spray.
Spoon 1 to 2 tablespoons of batter per mini donut onto the waffle iron, close, and cook for 2 to 4 minutes until golden.
Repeat with remaining batter until all mini donuts are cooked.
Whisk together powdered sugar, milk, and vanilla extract in a bowl until smooth to make the glaze. Adjust thickness with more milk or sugar.
Dip warm mini waffle donuts into the glaze or drizzle it over the top. Optionally, sprinkle with cinnamon sugar.