1. Preheat oven to 350°F. Beat first six ingredients on medium speed for 8 minutes. Pour half of the mixture into a greased 11x17. Mix filling/topping ingredients and sprinkle half over first half of mixture in pan. Pour in the rest of the mixture and top with the filling/topping. Swirl with a knife.
2. Bake for 20-30 minutes.
3. Beat frosting ingredients while baking the coffeecake. Add enough powdered sugar to make the frosting the consistency you prefer. Drizzle the coffee cake with frosting immediately after taking it from the oven. Cool completely before serving.