Preheat the oven to 400 degrees. Spray 12 medium muffin cups with cooking spray or line with paper baking cups.
In a large bowl, whisk together milk, oil, egg, and vanilla. In a smaller bowl, combine flour, sugar, baking powder, salt, and cardamom.
Stir into the wet ingredients until flour is moistened and just combined. Fold in the rhubarb.
Divide the batter evenly among the muffin cups.
In a small bowl, combine flour and brown sugar. Cut the butter into small pieces.
Use a pastry blender to cut the butter into the flour and sugar until crumbly.
In a pinch, forks or knives can be used. Top each muffin with streusel topping.
Bake for 20-25 minutes or until golden brown. Serve warm with a cup of coffee or tea.