2 tablespoons all-purpose flour
1 teaspoon cinnamon
2 tablespoons unsalted butter softened
½ cup packed light brown sugar
½ cup finely chopped pecans
½ cup (about 3 ounces) semisweet chocolate pieces
1¾ cups all-purpose flour
½ cup (scant) unsweetened cocoa powder
½ teaspoon baking powder
½ teaspoon baking soda
¼ teaspoon salt
2 sticks unsalted butter
1 cup superfine sugar (see note)
3 large eggs
1 teaspoon vanilla extract
1 cup sour cream
¼ cup milk or buttermilk
Heat oven to 350 degrees. Lightly grease a 9-by-9-inch baking pan.
For the streusel: Blend the flour and cinnamon with a fork. Then mash in the butter using a pair of butter knives or a pastry cutter until fine pea-shaped consistency. Work the sugar in with your fingers. Mix in the nuts and chocolate.
For the cake: Sift flour, cocoa, baking powder, baking soda and salt into a large bowl; set aside.
With an electric mixer on low, beat the butter 1 minute, or until light. Slowly add in the sugar about 2 tablespoons at a time. Continue beating on medium speed 4 minutes, scraping bowl as needed. Add eggs one at a time beating for 10 seconds between additions. Add vanilla and beat 1 minute.
In a medium bowl, blend the sour cream and buttermilk. With a large spatula, fold the dry ingredients into the batter in three additions, alternating with the sour cream in two of the additions. Beat the batter for 1 minute or until smooth.
Spread half the batter into the prepared pan and sprinkle it with half the streusel. Spread the remaining batter over the streusel and sprinkle top with remaining streusel. Bake 45-60 minutes until tester comes out dry. Cool cake to room temperature to serve.
Cook's notes: To make superfine sugar, blend granulated sugar in a food processor until it's superfine. If you don't have a 9-inch square pan, you can improvise in a 9-by-13-inch pan by fashioning a folded piece of aluminum foil to divide pan at 9-inches.