Begin by preheating your oven to 325 degrees F. Line about 18 muffins cups with paper liners. Set aside.
In a large bowl, stir together the rhubarb, brown sugar, buttermilk, vegetable oil, egg, and vanilla extract.
In a medium bowl, whisk together the flour, baking soda, and salt.
Add the dry ingredients to the wet ingredients and mix until just combined.
Spoon the batter into the prepared muffins tins. Fill each liner about 3/4 of the way full.
In a small bowl, mix together the sugar and melted butter until it resembles wet sand. Sprinkle this mixture in clumps onto the muffins.
Bake for 24-27 minutes or until a toothpick inserted near the center comes out clean.
Allow the muffins to cool in the pan for at least 10 minutes before removing.