In a large bowl, add the flour, sugar, salt, baking powder, and baking soda and whisk to combine.
In a medium bowl or liquid measuring cup, add the buttermilk, melted butter, and egg yolks and whisk to combine.
Add the buttermilk mixture to the dry ingredients and gently fold with a rubber spatula until just combined.
Add the egg whites and fold until just combined. Be sure not to overmix. Some lumps are okay.
Let the batter rest for 15-30 minutes at room temperature.
Add the butter to a cast iron skillet and heat over medium low heat. Once the butter has melted and is bubbling, reduce the heat to medium-low and add ⅓ cup (75 g) of batter to the pan.
Cook for 2-3 minutes, until bubbles start to appear on the surface. If adding any toppings like chocolate chips or blueberries, sprinkle over the pancake. Flip the pancake over and cook for another 1-2 minutes, until golden brown. Repeat with the remaining batter.