In a skillet over medium heat, melt butter. Add apple rings; sprinkle with 2 teaspoons of sugar. Cook until tender, turning once. In a bowl, beat the egg yolk, lemon zest and vanilla for 1 minute. In another bowl, beat egg white until stiff peaks form; fold in flour and remaining sugar. Fold into egg yolk mixture. Place apple rings in a greased 2-cup baking dish. Spread egg mixture on top. Bake at 350° for 15-18 minutes or until golden brown and set. Invert onto a serving plate.