In a small bowl, whisk together the sour cream and heavy cream. Refrigerate.
Put the flour in a bowl. Make a well in the center and add the egg yolks. Break them up and gradually whisk them in, along with the buttermilk, just until combined (there will still be a few small lumps); don’t overwhisk or the cakes will be tough. Add the salt and the 1/4 cup melted butter and whisk to combine; it will look like lumpy pancake batter. Stir in the herbs.
In a stand mixer fitted with the whisk attachment or with a hand mixer, whip the egg whites to soft peaks. Gently whisk about one-quarter of the whites into the batter to lighten it and then gently fold in the rest with a rubber spatula.
Heat 1 tsp. butter in an 8-inch nonstick skillet set over medium heat. Scoop about 1/3 cup batter into the middle of the pan and immediately nudge it with a heatproof rubber spatula toward the edge of the pan so it spreads to about 1/4 inch thick. When the first side is browned and crisp around the edges, after about 1 to 2 min., flip the matafan. Cook to brown and crisp the other side, 1 to 2 min. Wipe out the pan with a wadded paper towel and cook the remaining batter the same way.
Serve immediately, topped with a spoonful of the sour cream mixture, the crumbled bacon, and fresh herbs, if you want. You can hold the cooked matafans in a 200°F oven, layered between paper towels on a baking sheet, until you’ve cooked the entire batch, 20 to 30 min.