Step 1

Preheat the oven to 350 degrees Fahrenheit (175°C) and prep two 12-cup muffin pans. Either spray them with cooking spray or line them with paper cups.

Step 2

Make the streusel by mixing the sugar, melted butter, and cinnamon with a fork until it resembles coarse sand. Set aside.

Step 3

Sift together the flour, baking soda, baking powder, and salt in a medium bowl.

Step 4

In a separate large bowl, beat the brown sugar, buttermilk, oil, egg, and vanilla with an electric mixer until smooth.

Step 5

Pour the dry ingredients into the wet and mix with a rubber spatula until almost combined (some flour streaks should be visible).

Step 6

Add the rhubarb and walnuts and stir until just combined.

Step 7

Spoon the batter into the prepared pans, filling the cups 2/3 full.

Step 8

Sprinkle 1 teaspoon of streusel over each muffin cup, then bake for 25 minutes or until the muffin tops bounce back when lightly pressed.

Step 9

Let the muffins cool in the pans for 10 minutes before serving. Enjoy!