Buttermilk Pancakes

For the Dry Pancake Mix: Combine flour, baking powder, baking soda, salt, and sugar in a medium bowl and whisk until homogenous. Transfer to an airtight container. The mix will stay good for 3 months. (See note.) For each Batch of Pancakes: Place one batch of dry mix in a large bowl. In a medium clean bowl, whisk the egg whites until stiff peaks form. In a large bowl, whisk the egg yolks, buttermilk, and sour cream until homogenous. Slowly drizzle in the melted butter while whisking. Carefully fold in the egg whites with a rubber spatula until just combined. Pour the mixture over the dry mix and fold until just combined (there should still be plenty of lumps). Heat a large heavy-bottomed nonstick skillet over medium heat for 5 minutes (or use an electric griddle). Add a small amount of butter or oil to the griddle and spread with a paper towel until no visible butter or oil remains. Use a 1⁄4-cup dry measure to place 4 pancakes in the skillet and cook until bubbles start to appear on top and the bottoms are golden brown, about 2 minutes. Carefully flip the pancakes and cook on the second side until golden brown and completely set, about 2 minutes longer. Serve the pancakes immediately, or keep warm on a wire rack set on a rimmed baking sheet in a warm oven while you cook the remaining 3 batches. Serve with warm maple syrup and butter.