Preheat your oven to 350°F (175°C) and prepare your muffin tins by greasing or lining them.

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

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

Fold the dry mixture into the wet ingredients gently until just incorporated; do not overmix.

Gently fold in the chopped rhubarb and walnuts, if using, until evenly distributed.

Spoon the batter into prepared muffin cups, filling them about ¾ full.

Mix sugar, melted butter, and cinnamon for the streusel topping until crumbly; sprinkle over batter in muffin cups.

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

Let cool in the pan for about 10 minutes, then transfer to a wire rack to cool completely.