Preheat oven to 375°F (190°C). Grease and flour a 12-cup muffin tin, or line with paper liners.

In a large bowl, whisk together the flour, granulated sugar, baking powder, salt, and cinnamon for the muffins.

In a separate medium bowl, whisk together the egg, milk, vegetable oil, and vanilla extract.

Add the wet ingredients to the dry ingredients and stir until just combined. Do not overmix. Gently fold in the chopped rhubarb.

In a small bowl, combine the flour, brown sugar, and cinnamon for the crumb topping. Cut in the cold butter using a pastry blender or your fingertips until the mixture resembles coarse crumbs.

Divide the muffin batter evenly among the prepared muffin cups, filling each about two-thirds full. Sprinkle the crumb topping generously over the batter in each cup.

Bake for 20-25 minutes, or until a wooden skewer inserted into the center of a muffin comes out clean.

Let the muffins cool in the tin for 5-10 minutes before transferring them to a wire rack to cool completely.