Step 1Combine the sugar and cinnamon in a small dish and set aside.
Step 1Place a rack in the center of the oven; heat the oven to 375 degrees. Line 12 muffin cups with paper or spray with cooking spray. Set aside.
Step 2Whisk together the sugar, oil, buttermilk, egg, egg whites and vanilla in a large bowl until smooth.
Step 3Place the strawberries and rhubarb in another bowl with 1/2 cup of flour. Use your hands to toss until the fruit is well-coated. Add the remaining flour, cinnamon, baking powder, baking soda and salt to the egg mixture. Use a wooden spoon to combine, then add the fruit mixture. Mix well.
Step 4Put 1/4 teaspoon of Cinnamon Sugar in each muffin cup. Spread (or shake the pan) until the bottoms of the cups are evenly coated. Spoon about 1/4 cup of batter into each cup. Drizzle 1/2 teaspoon of butter over each; sprinkle 1/2 teaspoon Cinnamon Sugar over each.
Step 5Bake the muffins until they're well-browned and a toothpick inserted into the center of a muffin comes out clean, about 22 to 25 minutes. Cool the muffins in the pan for 30 minutes. (These can be completely cooled and frozen up to 1 month in an airtight plastic bag.)