Preheat oven to 350°.
To prepare cake, weigh or lightly spoon flour into a dry measuring cup; level with a knife. Sift together flour, cocoa, and 1/4 teaspoon salt.
Place 6 egg whites and 1/4 teaspoon cream of tartar in a large bowl; beat with a mixer at medium-high speed until soft peaks form. Add 1/2 cup sugar, 2 tablespoons at a time, beating until stiff peaks form (do not underbeat). Beat in vanilla. Sift flour mixture over egg white mixture, 1/4 cup at a time, folding flour mixture into egg white mixture. Spoon batter into an ungreased 10-inch tube pan, spreading evenly. Break air pockets in batter by cutting through batter with a knife. Bake at 350° for 20 minutes or until cake springs back when lightly touched. Invert pan; cool completely. Loosen cake from sides of pan using a narrow metal spatula. Invert cake onto plate.
Split cake horizontally into 3 even layers using a serrated knife. Working with 1 cake layer at a time, cut with a 3-inch round cutter into 4 rounds. Repeat procedure with remaining 2 cake layers; discard remaining cake, or reserve for another use.
Coat 6 (8-ounce) ramekins with cooking spray. Line each ramekin with plastic wrap, allowing the plastic wrap to extend over edges. Spoon 1/4 cup yogurt into bottom of each ramekin, spreading evenly; top each with 1 cake round. Spread 1/4 cup yogurt over each cake layer, spreading evenly; top each with 1 cake round. Cover with plastic wrap; freeze 4 hours or up to overnight.
To prepare meringue, combine 6 egg whites and remaining ingredients in top of a double boiler. Cook over simmering water 2 minutes or until candy thermometer registers 150°, stirring constantly with a whisk. Remove from heat. Beat egg mixture with a mixer at medium speed until soft peaks form; beat at high speed until stiff peaks form.
Invert ramekins, cake sides down, onto a baking sheet. Discard plastic wrap. Divide meringue evenly among servings, and spread evenly over each dome (the domes should be completely covered with meringue). Holding a kitchen torch about 3 inches from domes, heat the meringue, moving the torch back and forth until lightly browned. Transfer to individual plates, and serve immediately.