In large bowl, mix 1/2 cup oats and the buttermilk with fork; let stand 5 minutes. Stir in egg, brown sugar, butter, and carrot. Add flour, baking soda, and 1/4 teaspoon cinnamon, stir just until dry ingredients are moistened. Divide batter evenly among muffins cups.
In small bowl, mix 1 tablespoon oats, the granulated sugar, and 1/4 teaspoon cinnamon. Sprinkle over batter in each cup.
Bake 15 to 20 minutes or until toothpick comes out clean. Immediately remove from pan to wire rack. Serve warm.