In a large bowl, combine flour, baking powder, baking soda, ¼ cup granulated sugar, brown sugar and salt.
In a separate bowl, whisk sour cream, milk, oil, eggs, and vanilla extract.
Pour the wet ingredients into the dry ingredients and stir with a rubber spatula or wooden spoon until mostly combined. Add rhubarb and stir until just combined, being careful not to overmix.
Grease 12 standard muffin cups or line with paper liners. Divide the batter between the 12 cups, filling each about ¾ full.
In a small bowl, combine the melted butter, 2 tablespoons granulated sugar, pecans and ground cinnamon. Sprinkle evenly on top of the muffins.
Bake for 15-20 minutes, until a toothpick inserted into the center comes out clean. Place the pan on a wire rack to cool for 10 minutes, then carefully remove each muffin and place directly on the rack to cool completely.
Depending on how full you fill the muffins cups, you should get 12-16 muffins. If you have 12 count muffin pans, you can fill 8 muffins in each pan to keep them even. Pro-tip: if you have empty muffins cups, fill them halfway with water so the empty cup won't burn or warp during baking.