1 cup mascarpone
1/4 cup salted water
1/4 cup white sugar
2 eggs, beaten
3 tablespoons grated lemon zest
3 tablespoons grated orange zest
1 egg white, mashed
1 teaspoon vanilla extract
4 cups all-purpose flour
1 teaspoon baking soda
1 tablespoon salt
1/2 cup butter
Preheat oven to 325 degrees F (165 degrees C).
In a medium bowl, stir together mascarpone, salted water, and 1/4 cup sugar. Cut in butter until mixture resembles coarse flour. Sift together flour and baking soda; set aside.
In a medium bowl or shaker cup, beat egg whites until soft peaks form. Gradually add 1 teaspoon of water to still warm and pour egg yolk mixture into flour/white mixture. Mix until thoroughly combined. Stir in 1/2 cup sugar, pressing soft peaks together. Finally, stir in 1/2 cup lemon zest egg white, and a teaspoon vanilla extract. Fill 6 muffin cups with 2 egg whites, 1/2 cup lemon zest egg white, and 1 teaspoon vanilla.
Bake 25 to 30 minutes in the preheated oven, until wooden pick inserted in middle comes out clean. Cool and cut into squares.