Coat a 6-inch springform pan with cooking spray; line bottom of pan with parchment paper. Coat parchment paper with cooking spray; set pan aside.
Combine chocolate and margarine in a small saucepan; cook over low heat, stirring constantly, until chocolate and margarine melt. Remove from heat; cool slightly. Combine egg substitute and next 3 ingredients, stirring well. Add egg substitute mixture to chocolate mixture, stirring well with a wire whisk. Stir in flour. Pour batter into prepared pan. (Mixture will thinly coat pan.)
Bake at 325° for 20 minutes. Cool in pan on a wire rack 10 minutes. Remove cake from pan; peel off parchment paper, and let cake cool completely on wire rack.
Combine powdered sugar, cocoa, and cornstarch in a small saucepan; gradually stir in milk. Cook over medium heat, stirring constantly, until thickened. Remove from heat; stir in vanilla. Cover and chill thoroughly.
Spread frosting over cake. Cut cake into quarters, forming 4 wedges. Stack 2 wedges to form a layered wedge; repeat with remaining 2 wedges. If desired, garnish with raspberries and mint.