COMBINE egg yolk and sugar; beat with an electric mixer until thick and yellow. Add cream cheese, cottage cheese and vanilla; mix well. Refrigerate until ready to use.
COMBINE eggs, oil and milk in medium bowl; beat until well blended.
ADD flour and salt; continue to beat until batter is smooth and flour is dissolved. Refrigerate, covered, 30 minutes or until ready to use.
HEAT a small skillet slowly. To test temperature, drop a little cold water onto surface; water should bead up and bounce. For each blintz, brush pan lightly with melted butter.
MEASURE 3 Tbsp. batter into cup. Pour into skillet all at once, rotating skillet quickly to spread evenly.
COOK 1 minute until golden brown on underside; loosen edge with spatula and remove. Dry on paper towels. Stack, brown side up, with waxed paper between blintzes.
HEAT oven to 200ºF.
SPREAD 3 Tbsp. filling on browned side of each blintz, making a rectangle 4 inches long. Fold two sides in over filling, overlapping edges and covering filling completely.
MELT 1 Tbsp. butter in large skillet over medium heat. Add half of blintzes, seam-side-down and not touching. Sauté until golden brown, turning once. Keep warm in oven while cooking second half of blintzes.
SERVE hot, sprinkled with powdered sugar and topped with sour cream and preserves.