Line baking sheet with parchment paper. Draw twenty-four 2-inch circles on the parchment paper, about 1 inch apart; set aside.
In large mixer bowl, beat egg whites and lemon juice at medium speed until soft peaks form. Use back of small spoon to spread about 2 tablespoons meringue into 2-inch circles on prepared baking sheet, building up sides to form shells.
Bake in 250°F oven for 1 hour. Turn oven off; do not open door. Let meringue shells stand in oven overnight. Carefully remove shells from parchment. Store shells in airtight container at room temperature. Just before serving, spoon filling into shells. Garnish each tart with strawberry half or raspberry. If desired, drizzle with raspberry plate scapers.
French Silk Filling:.
In large mixer bowl, beat butter, powdered sugar, and cocoa powder at medium speed until creamy. Add melted chocolate and vanilla; beat at low speed until combined. Beat at high speed until light and fluffy, scraping bowl as needed. Add eggs, one at a time, beating well after each addition. Beat at medium speed until light and fluffy, about 2 to 3 minutes. (Filling can be made in advance and refrigerated. Let stand at room temperature for 30 minutes before spooning into tarts.) Just before serving, spoon into meringue shells. Serve immediately or chill for up to 2 hours.