Preheat oven to 400. Line a muffin tin with cupcake wrappers.
In a large bowl, mix together wet and dry ingredients for the muffins until well combined.
Gently stir in chopped apple.
Scoop about ¼ cup batter into each cupcake liner -- the cups should be ⅔ full. Set aside
In a medium bowl, mix together ingredients for streusel topping.
Spoon streusel topping on top of the batter in each muffin cup. Use as much as you want -- I sprinkled enough to cover the entire top.
Sprinkle a little cinnamon on top of the streusel.
Bake for 20-25 minutes.
Enjoy warm with a glass of milk.
Notes
The original recipe called for ¼ cup flour to be used for the streusel, but I found that the mixture was way too runny with just that amount. I kept adding more flour a little at a time until the topping became crumbly and was recognizable as streusel. ½ cup is my best guess at the total amount used.
Recipe by Just Putzing Around the Kitchen at https://www.justputzing.com/2011/09/apple-streusel-muffins.html