Bring milk to scalding in a small pan over medium heat; let cool.
In a large bowl, combine yeast, water, and sugar; let stand until bubbly.
In another bowl, combine flour, pancake mix, and salt; then add to yeast mixture along with milk, egg yolks, and sour cream.
Beat until smooth and well blended (about 3 minutes).
Cover and let stand at room temperature until mixture is spongy and has a slightly sour smell .
In a bowl, beat egg whites until soft, moist peaks form.
Gently fold into batter; let stand for 30 minutes.
Heat a griddle or wide frying pan over medium high heat; brush lightly with butter.
Ladle 3 tablespoons batter onto griddle for each pancake; cook until tops are dry and bubbles break .
Turn and brown other side.
Stack pancakes and keep them warm while preparing remaining pancakes.
Pass toppings at the table.
Toppings.
Offer 1/2 cup butter, melted; 1/2 pint (1 cup) sour cream; 1 mild red onion, thinly sliced; 4 hard cooked eggs, chopped; and fresh or dry dill weed.