Preheat the oven to 350 degrees F (175 degrees C). Grease two 12-cup muffin pans or line with paper cups.

To make the rhubarb muffins: Stir flour, baking soda, baking powder, and salt together in a medium bowl. Beat brown sugar, buttermilk, oil, egg, and vanilla in a separate bowl with an electric mixer until smooth. Pour in dry ingredients and mix by hand just until blended. Stir in rhubarb and chopped walnuts. Spoon batter into the prepared cups, filling almost to the top.

To make the streusel topping: Stir sugar, melted butter, and cinnamon together in a small bowl; sprinkle about 1 teaspoon streusel on top of each muffin.

Bake in the preheated oven until muffin tops spring back when lightly pressed, about 25 minutes. Cool in the pans for 10 minutes, then serve warm or transfer to a wire rack to cool completely.