In a large bowl, mix polenta, flour, baking powder, soda, sugar, and salt.
Add egg and buttermilk. Beat with a whisk until batter is smooth. Stir in onions, corn, and butter.
Place a griddle or 10- to 12-inch frying pan (nonstick, if desired) over medium heat. When it's hot, spoon batter in 1/4-cup portions onto griddle, spacing slightly apart. Cook until edges look dry, turn with a wide spatula, and brown bottoms, 2 to 3 minutes total.
Serve pancakes as cooked, or arrange in a single layer on racks on baking sheets and keep warm in a 150° oven up to 15 minutes.
Stack pancakes on plates and serve with chipotle sauce added to taste.