Heat oven to 350°F. Grease bottom only of 13x9-inch pan. Make brownies as directed on box for 13x9-inch pan. Cool completely, about 1 hour.
Meanwhile, place chocolate in medium bowl. In 1-quart saucepan, heat 3/4 cup of the whipping cream and the salt just to boiling; pour over chocolate. Let stand 4 to 5 minutes or until chocolate is melted and smooth when stirred. Stir in vanilla; let stand 15 to 20 minutes or until completely cool.
In chilled large bowl, beat 1 1/2 cups of the whipping cream with electric mixer on high speed until stiff. Stir about one-fourth of the whipped cream into cooled chocolate mixture; fold chocolate mixture into whipped cream. Spread evenly on top of brownies. Cover and refrigerate about 2 hours or until firm.
Beat remaining 1 1/2 cups whipping cream and the powdered sugar in chilled medium bowl with electric mixer on high speed until stiff peaks form. Spread evenly on top of chocolate mixture; garnish with chocolate curls. Cut into 6 rows by 4 rows.