Position a rack in the center of the oven and heat the oven to 400°F Line a 12-cup muffin tin with paper or foil baking cups.
Make the muffin batter: In a large mixing bowl, combine the flour, sugar, baking powder, cinnamon, baking soda, and salt and whisk to blend.
In a medium bowl, whisk together the sour cream, melted butter, eggs, and vanilla until smooth. Lightly stir the sour cream mixture into the dry ingredients with a spatula until the batter just comes together; do not overmix. Gently stir in the diced rhubarb. The batter will be thick.
Divide the batter among the muffin cups, using the back of a spoon or a small spatula to settle the batter into the cups. The batter should mound a bit higher than the tops of the cups.
Make the topping: In a small bowl, combine the sugar and cinnamon and mix well. Sprinkle a generous 1/2 teaspoons of the cinnamon-sugar mixture over each muffin.
Bake the muffins until they’re golden brown, spring back most of the way when gently pressed, and a pick inserted in the center comes out clean, 18 to 22 minutes. Transfer to a rack and let the muffins cool in the pan for 5 to 10 minutes. Carefully lift the muffins out of the pan—if necessary, loosen them with the tip of a paring knife—and let them cool somewhat. Best served warm.