Heat an electric griddle to 350ºF or set a heavy skillet or griddle over medium heat.
Heat oven to the lowest possible temperature, not more than 200ºF.
Place the pancake mix in a large mixing bowl.
In a second bowl, whisk together the egg whites and buttermilk.
Whisk the egg yolks into the cooled melted butter. Whisk the butter mixture into the buttermilk mixture until completely incorporated.
Pour the wet ingredients into the dry ingredients and quickly bring together with a large whisk. I allow myself only 12 stirs, then I walk away. Yes, there will be some lumps in the batter, and that's fine.
Check the griddle. When a drop of water skitters across the surface like a little hovercraft, it's ready. Lightly grease the griddle with butter, shortening, or nonstick spray, and wipe off the excess with a paper towel.
Ladle 1 ounce (by volume) of batter onto the griddle using either a #20 disher or a 1-ounce ladle. As soon as the batter hits the pan, use the underside of the ladle or disher to gently coax the batter out to a disk about 4 inches in diameter. Cook until bubbles around the upside edge set, about 3 minutes. If you wish to integrate the fruit into the pancake itself, add it after the batter has been on the griddle for about 30 seconds.
Carefully flipThe first cakes tend to be uglier than later ones, so feed the in-laws first. with a wide spatula and cook until golden brown on the bottom, another 2 to 3 minutes.
Serve immediately or place on a half-sheet pan, cover with a tea towel, and keep warm in the oven for up to 20 minutes.