Preheat your oven to 350°F (175°C) and prepare your muffin pans.

In a medium bowl, whisk together flour, baking powder, baking soda, and salt.

In a large mixing bowl, cream together brown sugar and buttermilk. Add vegetable oil, egg, and vanilla extract, mixing until smooth.

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

Carefully fold in the chopped rhubarb and walnuts.

Fill prepared muffin cups almost to the top with batter.

In a small bowl, mix together sugar, melted butter, and cinnamon for the streusel topping. Sprinkle over muffin batter.

Bake for approximately 25 minutes until golden and a toothpick comes out clean.

Let muffins cool in pans for about 10 minutes before transferring to a wire rack.