Preheat the oven to 350°F. Grease 12 muffin tins with coconut oil or use liners.
Mix egg, applesauce, coconut sugar, and vanilla until smooth. Slowly add melted coconut oil while stirring.
Add all dry ingredients (except diced apple and baking powder) and mix well. Gently fold in diced apple and baking powder. Use a large cookie scoop to fill each muffin tin ¾ full.
Make the crumb topping by combining all streusel ingredients. Sprinkle it on each muffin.
Bake at 350°F for 17-20 minutes or until a toothpick comes out with a few moist crumbs. Let them cool for at least 20 minutes before enjoying! Store in an airtight container for up to 2 days on the counter, 8 days in the fridge, or freeze for up to 3 months.