Preheat the oven to 375 degrees Fahrenheit. Line a 12-cup muffin pan with paper liners or grease lightly with nonstick spray.
In a large mixing bowl, whisk together the flour, baking powder, salt, and granulated sugar until evenly combined.
In another bowl, whisk the egg, vegetable oil, milk, and vanilla extract until smooth and creamy.
Pour the wet ingredients into the bowl of dry ingredients. Stir gently with a spatula until just combined. Do not overmix the batter.
Fold the chopped rhubarb into the batter until evenly distributed throughout the mixture.
Spoon the batter evenly into the prepared muffin cups, filling each cup about three-fourths full.
In a small bowl, mix together the cinnamon and sugar. Sprinkle the topping evenly over each muffin.
Bake for 20 to 25 minutes or until the muffin tops are golden brown and a toothpick inserted into the center comes out clean.
Allow the muffins to cool in the pan for 5 minutes before transferring to a wire rack to cool completely.