In a medium bowl, add all the ingredients, then use your fingers or a fork to mix until tiny crumbs are formed.
Sprinkle each muffin with a generous amount of streusel topping.
Bake 18-22 minutes, or until toothpick comes out clean.
Let cool in pan 10 minutes, then remove muffins to wire rack to cool completely.
Muffins are best served fresh from the oven, but can be stored for up to 5 days.