1. Sift the flour, sugar, and salt into a mixing bowl. Make a well in the center of the dry ingredients. Break the eggs into the well and mix the eggs with a fork until well combined.
2. Add the milk and buttermilk in a steady stream, whisking constantly, while drawing in the flour. The batter should have the consistency of thin cream. Let the batter rest for 30 minutes.
3. Remove the tough stems from the mushrooms (reserve for other use) and wipe the caps clean with a damp paper towel. Cut the mushrooms into 1/8-inch-thick julienne strips.
4. Melt 1 tbs butter in a sauté pan. Add the mushrooms and cook over medium heat for 2 minutes, stirring constantly. Remove the pan from the heat and season the mushrooms with salt, pepper and parsley. You should have about 1/2 cup. Add the seasoned mushrooms to the crêpe batter and stir well.
5. Heat a 7-inch nonstick skillet or crêpe pan over medium heat. Brush the pan lightly with a small amount of clarified butter to prevent the crêpes from sticking.
6. Ladle 1/3 cup batter into the pan and, working quickly, rotate that pan with a turn of your wrist so that the batter evenly coats the bottom. Cook the crêpe until it is set and golden, 45-60 seconds, then loosen it with a spatula and turn over. Cook until just set, about 10 seconds longer. The underside should be paler than the top side. Repeat with the remaining batter. If the crêpes begin to stick, add a bit more butter to the pan.
7. Stack the finished crêpes on a plate and cover until ready to use. To keep warm, place the plate, loosely covered with aluminum foil, over a saucepan of boiling water.
8. To serve hot, fold the crêpes in half and arrange them, slightly overlapping, on an ovenproof platter. Drizzle with melted butter, cover with aluminum foil, and place in a preheated 350 oven for 10 minutes. Dollop with sour cream and serve immediately.