In a medium bowl, stir the flour, cornmeal, baking powder, sugar, and salt. In a small bowl, whisk the milk, sour cream, and eggs. With a rubber spatula, gently stir the egg mixture into the flour mixture until just blended. Stir in the corn. Let sit for 10 to 15 minutes. Meanwhile, position a rack in the center of the oven and heat the oven to 200°F.
Pour the oil into a small, heavy frying pan, preferably cast iron, to a depth of 1/2 inch. Heat over medium heat until it’s hot enough that a small dollop of batter sizzles when added. With a spring-lever miniature ice cream scoop or a tablespoon, scoop up a ball of the batter and gently release it into the hot oil. Add three or four more balls of batter to the hot oil, taking care not to crowd the pan. Reduce the heat to medium low so that the fritters cook gently. When golden brown on the bottom and barely cooked around the top edge, after 1 to 2 minutes, use a slotted spatula to turn the fritters and cook until golden on the bottom, 1 to 2 minutes longer.
Transfer the fritters to a wire rack set over a baking sheet, sprinkle generously with salt, and keep warm in the oven. Continue to cook the remaining batter in small batches, adding more oil as needed to maintain the 1/2-inch depth. Serve right away with the salsa.