Combine the brown sugar, buttermilk, oil, egg and vanilla in a large bowl. Add the rhubarb. Stir together the flower, baking soda, baking powder and salt in a bowl. Add the flour mixture to the rhubarb mixture. Stir until blended.
Spoon the batter into 12 greased muffin cups, filling 2/3 full. Combine the granulated sugar, nuts, butter and cinnamon in a bowl. Sprinkle over the tops of the muffins. Bake at 400 degrees for 20 to 25 minutes or until golden brown.
Note: You may substitute other fruit for the rhubarb. Our favorite is raspberries.
Lighter version: Use 1 cup brown sugar, 1/4 cup oil, 1/2 teaspoon vanilla extract and 1 3/4 cups flour. For topping, use 2 tablespoons quick-cooking oats, 1/4 cup sugar, 3 tablespoons butter and 2 teaspoons cinnamon.