Preheat your oven to 375°F (190°C). Line your muffin tin or lightly grease it.
Mix dry ingredients: In a large bowl, whisk together flour, baking soda, baking powder, and salt.
Combine wet ingredients: In another bowl, mix the brown sugar, buttermilk, oil, egg, and vanilla until smooth.
Bring it all together: Pour the wet mixture into the dry ingredients and stir just until combined—don’t overmix!
Fold in rhubarb and walnuts: Gently stir until evenly distributed.
Scoop into muffin tin: Fill each cup about ¾ full.
Make the streusel: In a small bowl, stir sugar, melted butter, and cinnamon together until crumbly. Sprinkle evenly over muffins.
Bake: Pop them in the oven for 20–25 minutes or until a toothpick comes out clean.
Cool: Let them cool in the pan for 5 minutes, then transfer to a wire rack.