1Mix the flour, sugar and cornstarch together in a small bowl and set aside.
2Using an electric mixer set on high, beat the eggs, water, butter and oil together until light yellow. Reduce the speed to low and blend in the flour mixture all at once, just until the white disappears. Do not overbeat as this can make the crepes tough.
3Pre-heat a crepe pan over medium-high until a droplet of water sprinkled on the pan sizzles. Brush the hot pan with butter, coating it well.
4For each crepe, pour about 1/4 cup of batter into the pan and immediately tilt the pan so the batter, completely but lightly, coats the bottom (some batter may move up the pan sides a little). Cook about 30 seconds until the bottom of the crepe is golden brown (lift an edge to check). Shake pan to loosen crepe, then gently turn it over with a spatula, being careful not to tear it. Cook crepe on the underside about 15 seconds, just until it's set. Turn the crepes, light underside-up, onto a cooling rack.
5Refrigerate crepes for up to 2 days or freeze up to 1 month.
6Make the filling:
Stir filling ingredients in a bowl until blended. To fill each crepe, spoon 3 tablespoons of filling in the center on the underside of the crepe, then fold the edges over like an envelope: first the top edge, then the left side, the right side and finally the bottom edge. Be sure the filling is tightly enclosed.
7To fry the blintzes, melt butter in a large skillet over medium heat. Place the filled blintzes in the skillet, folded ends down. Fry blintzes until golden on both sides, turning once, about 5 minutes total. Serve hot with applesauce or sour cream.