In a large bowl, let egg whites stand at room temperature for 30 minutes. Meanwhile, cover a large baking sheet with parchment paper or foil. Draw twelve 2-inch circles, 3 inches apart, on the paper or foil; set aside.

For meringues: Preheat oven to 300 degrees F . In a small bowl, stir together the 2/3 cup sugar and the lemon peel; set aside. Add cream of tartar to egg whites. Beat with an electric mixer on medium speed until soft peaks form (tips curl). Add the sugar-peel mixture, 1 tablespoon at a time, beating on high speed until stiff peaks form (tips stand straight). Spoon egg white mixture into the circles on the prepared baking sheet, building up sides slightly.

Bake for 25 minutes. Turn off oven. Let meringues dry in oven with door closed for 1 hour. Remove from oven; cool completely on baking sheet.

For filling: In a small bowl, stir together the 4 teaspoons sugar and the cocoa powder. In another small bowl, stir together mascarpone cheese and vanilla. Stir in the cocoa mixture and enough of the milk to make of spreading consistency.

Spread filling into cooled meringues. Top with berries. If desired, garnish with additional lemon peel and/or unsweetened cocoa powder.

Make-Ahead Directions: Prepare as above through step 3. Transfer meringues to an airtight storage container. Store at room temperature for up to 1 week. To serve, prepare filling and serve as above.