NOTE: Prepare filling a few hours before making the cake so that it has time to set up and chill.
FILLING: In a small saucepan, combine sugar and cornstarch. Stir in two third cup water. Cook over medium-low heat, stirring constantly, until mixture becomes translucent, thickens and bubbles, 6 to 8 minutes. Cook 1 minute longer. Remove from heat.
In a small bowl, beat half of hot mixture into egg yolk, then stir back into saucepan. Cook, stirring, 1 minute longer. Stir in lemon juice and lemon zest. Pour into a small bowl. Press plastic wrap directly on surface. Refrigerate at least 3 hours, or until very cold.
CAKE: Preheat oven to 375 degrees. Coat a 10x15 inch jelly roll pan with cooking spray. Line bottom of pan with wax paper, trim off excess paper from edges and coat again with cooking spray. On a sheet of wax paper, combine flour, baking powder and salt.
In small bowl of electric mixer, beat egg whites with cream of tartar on high speed til firm peaks form; set aside. In large bowl of electric mixer, beat egg substitute until thick and creamy, about 1 minute; lower speed. Gradually add sugar, beating constantly, until mixture is very thick, about 2 minutes. Stir in one third cup water and vanilla. Add flour mixture, stirring until combined. Fold in beaten egg whites. Spread batter evenly in prepared pan.
Bake dozen to 13 minutes, or until cake just begins to pull away from the sides of the pan and springs back when lightly pressed with a small metal spatula.
Loosen cake around edges with a knife. Invert pan onto a clean kitchen towel dusted with 2 tb. powdered sugar; peel off wax paper. Gently roll up cake and towel together from short end. Place roll, seam side down, on a wire rack; let cool completely.
When cool, unroll cake carefully. Spread with Light Lemon Filling. To start rerolling, lift end of cake with towel. Place roll, seam side down on serving platter. Dust with remaining 1 tb. powdered sugar.