For the muffins:
2 cups gluten-free flour
3/4 cup granulated sugar
2-1/2 tsp. baking powder
1 tsp. ground cinnamon
1/2 tsp. baking soda
1/2 tsp. kosher salt
1 cup sour cream
8 Tbs. unsalted butter, melted and cooled slightly
2 large eggs
1 tsp. pure vanilla extract
2 cups 1/4-inch-diced rhubarb
For the topping:
3 Tbs. granulated sugar
1/2 tsp. ground cinnamon
1. 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.
2. Make the muffin batter: In a large mixing bowl, combine the flour, sugar, baking powder, cinnamon, baking soda, and salt and whisk to blend.
3. 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.
4. 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.
5. Make the topping: In a small bowl, combine the sugar and cinnamon and mix well. Sprinkle a generous 1/2 tsp. of the cinnamon-sugar mixture over each muffin.
6. 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. Serve warm.