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1
Preheat the oven to 350 F (180 C). Grease and floura 9x13 inch baking pan or dish.
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2
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.
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3
In a bowl, stir together the 1 1/2 cup all-purpose and the cake flours, cocoa powder, baking soda, baking powder, and salt.
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4
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.
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5
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.
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6
Spoon the batter into the prepared pan and spread evenly. Sprinkle evenly with streusel.
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7
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.
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8
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.
