Preheat the oven to 350 F (180 C). Grease and floura 9x13 inch baking pan or dish.
For streusel topping: stir together the 2/3 cup flour, brown sugar, and cinnamon in a small bowl. Using a pastry cutter or fingers, cut or rub n the butter until coarse crumbs form. Stir in chocolate chips until evenly distributed. Set aside.
In a bowl, stir together the 1 1/2 cup all-purpose and the cake flours, cocoa powder, baking soda, baking powder, and salt.
In another bowl, using an electric mixer on medium speed, cream together butter and sugar until light and fluffy. Add eggs, one at a time, beating thorougly after each addition.
Add dry ingredients in 2 or 3 increments, alternating with sour cream and vanilla. Using mixer on low speed, beat until smooth and fluffy, about 2 minutes.
Spoon the batter into the prepared pan and spread evenly. Sprinkle evenly with streusel.
Bake until topping is golden-brown, about 40-45 minutes. A toothpick inserted into the center should come out clean. Transfer pan to a wire rack and let cool completely. Cut into squares to serve.
Notes: If using a glass baking dish, reduce oven temperature to 325 F (165 C). To intensify chocolate flavor, add 1 1/2 teaspoons chocolate extract when beating in the sour cream and vanilla.