Preheat your oven to 350°F (175°C) and grease two 12-cup muffin pans.

In a medium bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt.

In a larger bowl, beat together the brown sugar, buttermilk, oil, egg, and vanilla extract until smooth.

Gently fold the dry mixture into the wet mixture until just combined.

Fold in the chopped rhubarb and walnuts until evenly distributed.

Fill each muffin cup nearly to the top with the batter.

In a small bowl, combine sugar, melted butter, and cinnamon for the crumb topping.

Sprinkle about 1 teaspoon of this mixture over each muffin before baking.

Bake for 25 minutes or until the tops spring back when pressed.

Cool in the pan for about 10 minutes before transferring to a wire rack.