Combine 1 cup sugar, lemon juice, 4 eggs, egg yolk, and lemon peel in heavy medium saucepan and whisk to blend; add butter. Whisk constantly over medium heat until mixture thickens and coats back of spoon thickly (do not boil), about 12 minutes. Pour mixture through strainer into medium bowl. Place bowl in larger bowl filled with ice water and whisk occasionally until lemon curd is cooled completely, about 15 minutes. Whisk egg white and remaining 2 tablespoons sugar in small metal bowl to blend. Set bowl over small saucepan of barely simmering water and whisk constantly until sugar dissolves and mixture is just warm to touch, about 1 minute (do not overheat or mixture will curdle). Immediately remove bowl from over water. Using electric mixer, beat egg white mixture until thick and glossy (texture will resemble marshmallow creme; volume of beaten egg white will be very small). Gently fold egg white mixture into cooled lemon curd. DO AHEAD: Can be made 1 day ahead. Cover and chill.