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Preheat oven to 375 degrees. Line a 12-cupmuffin tin with baking cups, and lightlycoat each with cooking spray.
Whisk together cornmeal, sugar, rice flour, potato starch, tapioca starch, baking powder, and 1/2 teaspoon salt in a large bowl. Whisk together buttermilk, oil, and eggs in a medium bowl. Fold buttermilk mixture into cornmeal mixture, thengently fold in 1 cup grapes until combined.
Spoon batter into baking cups, fillingeach three-quarters full. Top with remaining1/2 cup grapes, and sprinkle with remainingtablespoon sugar. Bake until a toothpick insertedinto the center of each comes outclean, about 25 minutes. Transfer pan to awire rack, and let cool for 10 minutes. Turnout muffins onto rack. Let cool completely.