Place the egg whites in a large bowl. Let stand at room temperature for 30 minutes.
Meanwhile, line a 15x10x1-in. baking pan with parchment paper; lightly spray paper with cooking spray and set aside.
Preheat oven to 350ºF. Add vanilla and cream of tartar to egg whites. Beat on medium speed until soft peaks form. Gradually beat in sugar, 2 tablespoons at a time, on high until stiff glossy peaks form and sugar is dissolved.
Fold in flour, about 1/4 cup at a time. Spread batter into prepared pan. Bake for 15-20 minutes or until cake springs back when lightly touched. Cool 5 minutes.
Turn cake onto a kitchen towel dusted with powdered sugar. Gently peel off parchment paper. Roll up cake in the towel jelly-roll style, starting with the short side. Cool completely on a wire rack.
In a large saucepan, combine sugar and cornstarch. Whisk in water until smooth. Cook and stir over medium-high heat until thick and bubbly. Continue cooking for 1 minute. Remove from heat.
Crack your egg in a small bowl and lightly beat. Stir a small amount of the hot mixture into the egg and slowly add the egg mixture to the return all to the pan, stirring constantly. Bring to a gentle boil and pull off heat. Gently stir in lemon juice and peel. Let cool down for a few minutes, then fold in whipped cream a little bit at a time.
Unroll cake carefully and spread filling to within 1/2" of edges. Roll up again. Place seam side down on a serving plate. Sprinkle with powdered sugar, slice and serve! Enjoy!