Preheat oven to 375 degrees. Add muffin liners to a standard tin and set aside.
Beat brown sugar, oil, egg, buttermilk and vanilla in a mixing bowl.
Stir in rhubarb and walnuts with a spoon until combined.
In a separate bowl, whisk flour, baking soda, baking powder and salt.
Add the dry ingredients to the wet and stir with a spoon until just blended. Do not overmix.
Distribute batter evenly among muffin liners.
If you’d rather skip the icing, you can make a crumble topping by combining a tablespoon of melted butter, 3 tablespoons of brown sugar and a teaspoon of cinnamon. Sprinkle on muffin tops prior to baking.
Bake for 15 to 20 minutes or until muffins are golden brown and a tester placed in the center comes out clean.
Keeping the muffins in the pan, place on a wire rack to cool completely.
Frosting-free muffins can be stored at room temperature up to 3 days.
Store rhubarb cream cheese frosted muffins in an airtight container in the refrigerator up to 3 days.
Muffins can be frozen up to 3 months if stored in an airtight container or freezer bag.