Add all of the streusel ingredients to a mixing bowl and stir well to combine.
Set aside to dry out as you prepare the muffins.
To make the muffins:
Preheat oven to 400 degrees. Line a 12 cup muffin tin with muffin liners.
Add the flour, sugar, baking powder, cinnamon, and salt to a mixing bowl and stir to combine. Make a well in the center.
Whisk together the egg, milk, oil, and vanilla in a small bowl. Pour the mixture into the well and stir together with a fork. Do not overmix. The batter should be a bit lumpy.
Use a large cookie scoop to scoop the muffin batter into the prepared muffin tin. Sprinkle with the streusel.
Bake for 25 minutes or until a tester comes out clean.
Cool for 20 minutes before glazing.
To make the glaze:
Whisk together the powdered sugar and cream, adding additional powdered sugar or cream to reach the consistency you prefer.