Melt margarine in a large saucepan over medium-high heat; add brown sugar, stirring with a wire whisk. Remove from heat; cool slightly. Add egg substitute and buttermilk, stirring well.
Combine flour, cocoa, and salt; add to buttermilk mixture, stirring until blended. Stir in vanilla. Pour into a 9-inch springform pan lightly coated with cooking spray.
Bake at 350° for 15 minutes. Cool completely in pan on a wire rack.
Spread half of vanilla yogurt over brownie; spread chocolate yogurt over vanilla yogurt, and top with remaining vanilla yogurt. Cover and freeze 8 hours or overnight. Serve with chocolate syrup; garnish, if desired.
