Step 1: For the Crumb Topping: In a small bowl, combine 1/2 cup flour, 1/4 cup granulated sugar, 1/4 cup brown sugar, and 1/2 teaspoon cinnamon. Whisk together.
Step 2: Add 1/4 cup cold, cubed butter to the dry ingredients. Use your fingertips or a pastry blender to work the butter into the mixture until coarse crumbs form. Place in the refrigerator while preparing the muffin batter.
Step 3: For the Muffins: Preheat oven to 400°F (200°C). Line a 12-cup muffin tin with paper liners or grease well.
Step 4: In a large bowl, whisk together 1 1/2 cups flour, 1 teaspoon baking powder, 1 teaspoon baking soda, 1/2 teaspoon cinnamon, and 1/4 teaspoon salt.
Step 5: In a separate medium bowl, whisk together 1/2 cup granulated sugar, 1/4 cup brown sugar, 1 large egg, 1/2 cup buttermilk, 1/4 cup vegetable oil, and 1 teaspoon vanilla extract until smooth.
Step 6: Pour the wet ingredients into the dry ingredients. Mix with a spatula or wooden spoon until just combined. Do not overmix; a few lumps are okay.
Step 7: Gently fold in the 1 1/2 cups of chopped rhubarb until evenly distributed.
Step 8: Divide the batter evenly among the 12 muffin cups, filling each about two-thirds full.
Step 9: Retrieve the crumb topping from the refrigerator and generously sprinkle it over the top of each muffin.
Step 10: Bake for 18-22 minutes, or until the tops are golden brown and a wooden skewer inserted into the center comes out clean (a little melted rhubarb juice is fine).
Step 11: Remove from the oven and let cool in the muffin tin for 5 minutes before transferring to a wire rack to cool completely.