Preheat the oven to 375°F.
Line a 12-cup muffin pan with paper liners, or grease it well.
Stir the topping ingredients together in a small bowl.
Set the topping aside.
In a large bowl, whisk the flour, granulated sugar, brown sugar, baking powder, baking soda, salt, and cinnamon.
Break up any brown sugar lumps.
In another bowl, whisk the eggs, sour cream, milk, oil, and vanilla until smooth.
Pour the wet mixture into the dry mixture.
Stir with a spatula until the flour almost disappears.
Stop mixing before the batter looks completely smooth.
Add the diced rhubarb.
Fold the rhubarb gently into the batter.
Divide the batter among the muffin cups, filling each one about three-fourths full.
Sprinkle the cinnamon sugar topping over each muffin.
Bake for 18 to 22 minutes.
Start checking the muffins at 18 minutes.
Bake until the tops look set and lightly golden.
Check that a toothpick comes out clean or with a few moist crumbs.
Let the muffins rest in the pan for 5 minutes.
Move the muffins to a wire rack.
Let the muffins cool for about 15 minutes before slicing or serving.