Prepare crumb topping by mixing all ingredients together till crumbly; set aside.
Spray a muffin pan with non stick spray, and preheat the oven to 375°.
Place rhubarb in a small bowl; set aside.
Measure flour into a large mixing bowl. Scoop out about a tablespoon of flour and toss it with the rhubarb.
To the remaining flour, whisk in the sugar, baking powder, salt, and cinnamon.
In a small bowl, whisk together the egg, milk, oil, and vanilla.
Gently stir the wet ingredients into the flour mixture.
Carefully fold the rhubarb (and any excess flour in the bowl) into the batter.
Fill prepared muffin cups about 2/3 full, then divide the streusel topping evenly over the muffins.
Bake at 375° for 17-20 minutes or until a toothpick comes out clean. Let muffins sit in the pan for 3-5 minutes, then remove to cooling racks.