In a medium bowl, whisk the flour with the eggs, salt and 2 cups of water until smooth.
Make the filling
In a medium bowl, combine the ricotta with the egg yolks, granulated sugar, and vanilla extract.
Assemble the blintzes
Lay out two or three tea towels on your work surface.
Lightly brush an 8-inch nonstick skillet with a bit of clarified butter or oil and heat over medium until a drop of water sizzles.
Using a ¼ measuring cup, fill with 3 tablespoons of batter and pour into the heated pan, swirling the pan so the batter evenly coats it. Cook over moderate heat until just cooked and the center is dry to the touch, about 1 minute. Flip onto one of the tea towels and repeat the process until you have all 18-20 blintz pancakes. Let cool completely.
Fill the blintzes by placing a heaping tablespoon of the filling about 1-inch from the bottom of the pancake. Fold up the bottom and fold in the sides. Then roll up the blintz from the bottom like a burrito. Place the filled blintzes seam side down on a platter and refrigerate until ready to fry. (I find that allowing them to chill for at least an hour or overnight helps keep them sealed nicely.)
When ready to fry, heat ⅓ cup of neutral oil or clarified butter in a deep nonstick skillet over moderate heat. Turn them once or twice with tongs, careful not to pierce the outside, until they're nicely golden brown and crispy on both sides. Transfer to a paper towel-lined platter to drain, and, if necessary, keep them warm in a low oven until ready to serve. Serve with fresh fruit and a dollop of sour cream and dust with powdered sugar, if desired.