Preheat oven to 350F and line a 12 cup muffin pan with paper muffin liners.
Make the streusel first: combine the ingredients and rub together with your fingers until the butter is incorporated and the mixture has a dry crumbly texture. Set aside.
In a large mixing bowl, whisk together the oil, sugar, eggs, milk, and vanilla extract.
Next, whisk in the baking powder, baking soda and salt, making sure to mix well to get it evenly incorporated.
Whisk in the flour, and then gently fold in the chopped rhubarb.
Divide the batter evenly between the 12 muffin cups, and sprinkle streusel evenly over the top of the muffins.
Bake for 23-25 minutes.
Serve warm with butter!
Video
Notes
To refresh a leftover muffin, microwave it for 15-20 seconds. It's like it just came out of the oven!