In a large bowl, sift together the flour, baking powder, kosher salt, and granulated sugar. Set aside.
In a medium bowl, whisk together the buttermilk, large egg, and melted butter until the mixture is smooth and well combined.
Pour the wet ingredients into the bowl with the dry ingredients. Gently stir with a spatula or wooden spoon just until everything is incorporated. The batter will be slightly lumpy; do not overmix. If the batter is too thick to pour easily, add a little more buttermilk and stir gently.
Spray a nonstick skillet or griddle with cooking spray and place it over medium heat (about 300°F if using a griddle).
Once the pan is hot, use a ⅓-cup measuring cup to pour the batter into the pan. Spread it gently into a 4-inch circle if needed.
Cook the pancake until bubbles form on the surface and begin to pop, and the edges look set.
Flip the pancake carefully and cook until the other side is golden brown and the center is cooked through.
Wipe out the pan with a paper towel between batches, reapply cooking spray, and repeat with the remaining batter.
Serve warm with maple syrup.