Preheat oven to 375 degrees. Line a 12-cup
muffin tin with baking cups, and lightly
coat 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, then
gently fold in 1 cup grapes until combined.
Spoon batter into baking cups, filling
each three-quarters full. Top with remaining
1/2 cup grapes, and sprinkle with remaining
tablespoon sugar. Bake until a toothpick inserted
into the center of each comes out
clean, about 25 minutes. Transfer pan to a
wire rack, and let cool for 10 minutes. Turn
out muffins onto rack. Let cool completely.