Aunt Norma's Rhubarb Muffins

INGREDIENTS

• 2 1/2 cups flour
• 1 teaspoon baking soda
• 1 teaspoon baking powder
• 1/2 teaspoon salt
• 1 1/4 cups brown sugar
• 1/2 cup vegetable oil
• 1 egg
• 1 teaspoon vanilla extract
• 1 cup buttermilk
• 1 1/2 cups diced rhubarb
• 1/2 cup chopped walnuts
• 1 tablespoon melted butter
• 1/3 cup white sugar
• 1 teaspoon ground cinnamon