Preheat the oven to 375 degrees F. Grease a 10-inch cake pan with the tablespoon of butter, and lightly dust with flour. Set aside. In a large mixing bowl, cream the butter and sugar with a mixer on high speed. Into a separate bowl, sift all the dry ingredients. Add dry ingredients in 3 stages into the butter, alternating with the eggs 1 at a time, beating after the addition of each. Fold the sour cream into the batter. Pour the batter into the prepared cake pan, and arrange the berries evenly over the top. To make the streusel topping, place all the ingredients in a mixing bowl and blend. Evenly sprinkle the topping over the top of the berries. Bake in the oven until golden brown and risen, and a tester inserted into the center comes out clean, 40 to 45 minutes. Let cool 15 minutes before serving.