In a large bowl, whisk together the all-purpose flour, baking powder, baking soda, salt, and granulated sugar until evenly combined.
In a medium bowl or liquid measuring cup, whisk together the sourdough starter (or discard), milk, beaten eggs, and melted unsalted butter until smooth.
Pour the wet ingredients into the dry ingredients and stir gently until just combined — the batter should still be a little lumpy. Do not overmix.
Either cook the pancakes immediately, or cover the batter and leave it on the counter overnight to ferment for a stronger sourdough flavor.
Preheat a griddle or large skillet to 325°–350°F and spray lightly with cooking spray.
Pour 1/3 cup of batter onto the preheated griddle for each pancake. Cook until bubbles form on the surface and the edges look set, about 2 minutes.
Flip the pancakes and cook an additional 1–2 minutes, or until the bottoms are golden brown and cooked through.
Remove pancakes from the griddle, serve with your favorite toppings, and enjoy.