Preheat oven to 350°. Grease 1 (9-inch) round cake pan and 1 (9-inch) springform pan, and set aside.
Combine chocolate and butter in a large saucepan over low heat; cook, stirring frequently, until melted. Let cool to room temperature.
Stir sugar, eggs, and vanilla into chocolate mixture. Combine flour, baking powder, and salt. Stir into chocolate mixture.
Divide batter evenly between prepared pans. Bake 14 minutes or until toothpick inserted in center emerges with moist crumbs clinging. Cool completely on a wire rack.
Pour half of Simple Butterscotch Sauce over cake baked in springform pan. Spoon 4 cups ice cream evenly over butterscotch sauce. Remove cake from round cake pan, and place on top of ice cream, pressing gently. Pour over remaining butterscotch sauce; top evenly with remaining ice cream. Freeze 3 hours or until firm.
Run a knife around edge of springform pan, and release cake. Place on a serving plate. Pour Simple Hot Fudge Sauce on top of cake; sprinkle with nuts.