Preheat oven to 425°F. Line a muffin pan or grease well.
Make the crumb topping and set aside.
Mix dry ingredients in one bowl. In another, beat butter and sugars. Add eggs, yogurt, and vanilla.
Gradually mix in dry ingredients and milk until just combined. Fold in apples.
Fill muffin cups to the top. Add crumb topping and press gently.
Bake at 425°F for 5 minutes, then reduce heat to 350°F and bake for 15–18 more minutes.
Cool slightly, then drizzle with icing if using.