1. Preheat oven to 350°. Prepare Streusel Topping.
2. Beat butter at medium speed with an electric mixer until creamy; gradually add granulated sugar, beating well. Add eggs, 1 at a time, beating until blended after each addition.
3. 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-inch springform pan; top with sliced peaches and blackberries. Pinch off 1-inch pieces of Streusel Topping, and drop over fruit.
4. Bake at 350° for 1 hour and 10 minutes to 1 hour and 20 minutes or until center of cake is set. (A wooden pick inserted in center will not come out clean.) Cool completely on a wire rack (about 1 1/2 hours). Dust with powdered sugar. Garnish, if desired.
Note: We found that using a shiny or light-colored pan gave us the best results. If you have a dark pan, wrap the outside of the pan with heavy-duty aluminum foil to get a similar result.
Peach Coffee Cake: Omit blackberries. Increase peaches to 3 cups sliced (about 3 large peaches, 7 oz. each). Proceed with recipe as directed.