Preheat oven to 350.
Melt 2 tbsp butter in large skillet over medium-high heat; add apples and saute for 5 minutes or until softened. Remove from heat; cool completely (about 30 minutes).
Meanwhile, prepare streusel topping and caramel sauce. Set aside 1/2 cup caramel sauce for another use.
Caramel sauce: Bring all ingredients to a boil in a medium saucepan over medium-high head, stirring constantly; boil, stirring constantly for 2 minutes. Remove from heat and cool 15 minutes before serving. Store in airtight container in refrigerator for up to 1 week.
Streusel Topping: Mix all ingredients together in a medium bowl. Blend well. Let stand 30 minutes or until firm enough to crumble into small pieces.
Cake: Beat 1/2 cup butter on medium with electric mixer until creamy; gradually add sugar, beating well. Add eggs, 1 at a time, beating until blended after each addition.
Combine flour, baking powder and salt; add to butter mixture alternately with milk, beginning and ending with flour mixture. Beat at low speed until blended after each addition. Stir in vanilla. Pour batter into a greased and floured 9" springform pan; top with apples. Drizzle with 1/2 cup caramel sauce; sprinkle with streusel topping.
Bake cake at 350 for 45 minutes. Cover loosely with foil to prevent excessive browning; bake 25-30 minutes longer or until center is set. (A wooden toothpick will not come out clean). Remove from oven and cool in pan on wire rack for 30 minutes. Remove sides of pan. Cool completely on wire rack about 90 minutes. Drizzle with 1/2 cup caramel sauce.