Heat oven to 350°F. Line a 9-inch square baking pan with aluminum foil, extending foil 2-inches over edge.
Combine butter and chocolate in bowl. Microwave on Medium (50% power) 1 minute; stir. Continue microwaving, stirring every 15 seconds, 1-2 minutes or until butter and chocolate are melted and mixture is smooth.
Combine chocolate mixture and sugar in bowl; beat at medium speed until well mixed. Add eggs and vanilla; continue beating until well mixed. Add flour, cocoa and salt; beat at low speed until just combined.
Spread batter into prepared pan. Bake 15-19 minutes or until toothpick inserted in center comes out clean. (Do not overbake.) Cool completely.
Spread ice cream in even layer over cake; freeze 2 hours or overnight until ice cream is firm.
Combine sugar and egg whites in top of double boiler. Cook over medium-high heat, stirring constantly with whisk, 3-5 minutes or until mixture reaches 160°F. Pour into bowl; beat at high speed 8-10 minutes or until stiff peaks form.
Line baking sheet with aluminum foil. Remove cake and ice cream from freezer. Lift out of pan using foil ends. Cut cake into 9 squares. Work with 2 squares at a time (keeping remaining squares frozen). Place squares onto individual 8-inch squares of parchment paper or aluminum foil on cutting board. Top each cake square with 1/3 cup meringue. Spread meringue evenly over cake to cover all edges. Lift ends of parchment paper to move squares onto prepared baking sheet; place baking sheet in freezer. Repeat with remaining cake squares. Freeze, uncovered, for at least 4 hours. Cover with aluminum foil; freeze up to 2 days.
Heat oven to 500°F.
Remove baking sheet from freezer. Remove foil; place into oven. Bake 3-4 minutes or until meringue tips are golden brown. Serve immediately.