How to Make It
Preheat oven to 350°. Melt 2 Tbsp. butter in a large skillet over medium-high heat; add apples; sauté 5 minutes or until softened. Remove from heat; cool completely (about 30 minutes).
Meanwhile, prepare Streusel Topping and Caramel Sauce.
Whisk together flour, baking powder, and salt in a medium bowl.
Beat remaining 1/2 cup butter at medium speed with an electric mixer until creamy; gradually add sugar, beating well. Add eggs, 1 at a time, beating until blended after each addition. Add flour mixture alternately with milk. Beat at low speed until blended after each addition. Stir in vanilla.
Pour batter into a greased and floured 9-inch springform pan; cover top of batter with apples. Drizzle with 1/2 cup Caramel Sauce (reserve the rest); sprinkle with Streusel Topping.
Bake at 350° for 45 minutes. Cover loosely with aluminum foil to prevent excessive browning; bake 25 to 30 more minutes, or until center is set. (A wooden pick will not come out clean.) Cool in pan on a wire rack 30 minutes; remove sides of pan. Cool completely on wire rack (about 1 1/2 hours). Drizzle top of cake with remaining 1/2 cup Caramel Sauce before serving.