Preheat oven to 350°F (175°C) and lightly grease a 12-cavity donut pan.
In a large bowl, whisk pancake mix, brown sugar, cinnamon, and nutmeg.
In another bowl, whisk milk, egg, and melted butter.
Stir wet ingredients into dry ingredients until just combined.
Gently fold in the finely diced apples.
In a small bowl, mix crumb topping ingredients (except powdered sugar) until crumbly.
Spoon batter into donut pan, filling ¾ full.
Top each donut generously with crumb mixture.
Bake for 12–15 minutes or until a toothpick comes out clean.
Cool in pan 5 minutes, then transfer to a wire rack.
Once cool, sprinkle with powdered sugar.