Combine and mix well: brown sugar, oil, egg and vanilla. Stir in rhubarb, buttermilk and chopped nuts.
Combine in separate bowl: flour, baking powder, baking soda, salt, ginger and 1/2 teaspoon cinnamon. Add dry ingredients to rhubarb mixture and stir just to moisten. Fill muffin tins.
Combine white sugar, 1-1/2 teaspoon cinnamon and butter to topping. Sprinkle topping over batter in tins. Press gently into batter. Bake for 15 to 20 minutes in a 375°F oven.
muffin, rhubarb muffin