Whisk together flour and beer in a large bowl until it becomes smooth mass without lumps, salt to taste, season with pepper. Separate the yolk from the white. Add yolk into batter, let the dough stand for about 15 minutes.Melt butter, cool it, stir with a whisk into the batter. Beat up the whites to make a thick foam , gently pour in the batter. Slice fish fillet,dry with paper towel, salt and pepper to taste. Heat the oil for deep frying to 170°. Dip fish in batter and gently lower it into hot oil, fry for about 5 minutes and place it on a paper towel to soak the excess fat.