Grease or line a 12 cup muffin tin, set aside. Preheat oven to 425F.
In a small bowl whisk together flour, baking powder, baking soda and salt; set aside.
In a medium bowl whisk the sugar and egg together. Add in sour cream, milk, butter and vanilla and whisk until smooth. Add in the dry ingredients and whisk until batter has no lumps remaining. Stir in the rhubarb.
Fill the muffin tins with the batter, completely full.
To make the topping, stir the sugar, flour, cinnamon and salt together. Add in the butter and stir with a fork until the mixture is crumbly.
Evenly distribute the topping over each muffin, pressing the topping down onto the batter to ensure it sticks.
Bake for 5 minutes at 425F and then, leaving the muffins in the oven, lower the temperature to 350F. Bake for another 18-20 minutes.
Store muffins in an airtight container at room temperature for up to 3 days.