To prepare yogurt, spoon yogurt onto several layers of heavy-duty paper towels; spread to 1/2-inch thickness. Cover with additional paper towels; let stand 5 minutes. Scrape into a bowl using a rubber spatula. Combine yogurt and next 5 ingredients (through 1/8 teaspoon pepper) in a small bowl. Cover and chill.
To prepare fritters, heat a large nonstick skillet over medium-high heat. Coat pan with cooking spray. Add leek, 2 tablespoons water, 1/4 teaspoon salt, and 1/4 teaspoon pepper. Sauté 10 minutes or until golden, adding remaining 1/4 cup water as necessary to prevent leeks from sticking to pan. Place leeks and potatoes in a large bowl. Combine 1/2 teaspoon salt, 1/4 teaspoon pepper, eggs, and egg whites; stir well with a whisk. Add egg mixture to potato mixture.
Heat 4 teaspoons oil in a large nonstick skillet coated with cooking spray over medium-high heat. Spoon about 3 tablespoons potato mixture for each of 8 fritters into pan. Cook 3 minutes on each side or until browned. Remove from pan. Repeat procedure with the remaining 4 teaspoons oil and potato mixture. Sprinkle fritters with remaining 1/4 teaspoon salt and remaining 1/4 teaspoon pepper. Serve with yogurt sauce.