Grease a 9x9x2-inch baking pan; set aside. For streusel topping: In a large bowl, stir together the 2/3 cup flour, oats, brown sugar and cinnamon. Using a pastry blender, cut in the 1/3 cup butter until mixture resembles coarse crumbs. Stir in chocolate pieces and dried cherries. Set aside.
In a medium bowl, stir together the 1 cup flour, whole wheat flour, baking powder, baking soda and salt; set aside. In another large mixing bowl, beat the 1/2 cup butter with an electric mixer on medium to high speed for 30 seconds. Add granulated sugar and beat until fluffy. Add eggs and sour cream; beat until combined.
Add flour mixture and beat on low speed until just combined. Spread half of the batter into prepared pan. Sprinkle with half of the streusel mixture. Spoon remaining batter in mounds over streusel and spread to edges. Top with remaining streusel topping. Cover and refrigerate for 4 to 24 hours.
Uncover. Let stand at room temperature 30 minutes. Bake in a 350°F oven for 50 to 55 minutes or until golden and a toothpick inserted near the center comes out clean. Serve warm.