Preheat oven to 375 degrees F (190 degrees C). Grease 12 muffin cups, or line with paper muffin liners.
In a large bowl, stir together 1 cup flour, baking powder, salt, and white sugar. Make a well in the center and pour in oil, egg, and vanilla. Mix until just blended.
Stir in rhubarb. Spoon batter into prepared muffin cups.
In a small bowl, mix together brown sugar, 1/4 cup flour, melted butter, and cinnamon. Sprinkle over muffins.
Bake in preheated oven for 25 to 30 minutes, or until a toothpick inserted into center of a muffin comes out clean.
Interesting Facts
Rhubarb is often referred to as the "pie plant" because it is commonly used in pies.
Rhubarb is a vegetable, but it is used as a fruit because of its sweet taste.