Simmer cream, butter, and corn syrup in medium saucepan.Remove from heat. Add chocolate; whisk until smooth. Whisk in vanilla and salt. Let stand at room temperature until thick enough to spread, 1 hour to 1 1/2 hours. DO AHEAD: Can be made 4 hours ahead. Let stand at room temperature.
Transfer 1/2 cup buttercream to pastry bag fitted with medium star tip; reserve for piping rosettes. Brush off any crumbs from meringue layers and cake layers. Place several small dabs of buttercream down center of 15-inch-long rectangular serving platter. Place 1 meringue layer, top side up, atop buttercream on platter, anchoring to platter. Place strips of foil around meringue layer to protect platter when spreading meringue with buttercream. Using small offset spatula, spread 2/3 cup buttercream over meringue. Top with 1 cake layer, top side down, pressing gently to adhere. Brush cake layer with 1/3 cup peppermint syrup (cake layer will be very moist). Spread 2/3 cup buttercream over, then top with second meringue layer, top side up. Spread 2/3 cup buttercream over. Cover with second cake layer, top side down. Brush with 1/3 cup peppermint syrup (cake layer will be very moist). Spread with 2/3 cup buttercream. Top with third meringue layer, top side up. Spread thin layer of buttercream over top and sides of cake. Chill 15 minutes.
Spread remaining buttercream over top and sides of cake. Pipe small rosettes of buttercream in 2 rows atop center of cake. Stand 1 or 2 mint wafer triangles in each rosette. Chill cake at least 6 hours. DO AHEAD: Can be made 2 days ahead. Cover loosely; chill. Let cake stand at room temperature 1 1/2 hours before serving.
Garnish top of cake with fresh raspberries and mint leaves and serve.