Buttermilk replaces some of the usual butter in this recipe, producing a lighter texture. Before using buttermilk, always shake it well.
Preheat oven to 375 degrees. With an electric mixer, beat together butter and brown sugar in a large bowl until smooth. Beat in egg; add buttermilk, and beat to combine.
In a medium bowl, whisk together flour, baking powder, baking soda, and salt. Add to butter mixture; beat until smooth. Fold in rhubarb.
Coat a standard (12-cup) muffin tin with cooking spray. Spoon mixture into prepared tin, dividing evenly among cups. Sprinkle with granulated sugar.
Bake until a toothpick inserted in center of a muffin comes out clean, about 25 minutes. Cool in pan on a wire rack 15 minutes. Turn out muffins onto rack; cool completely.