By Doris Schechter, My Most Favorite Food 
For the Streusel
¾ cup packed brown sugar
¼ cup all purpose flour
1-½ tablespoons unsweetened cocoa powder
¼ teaspoon cinnamon
4 tablespoons chilled unsalted butter, cut into pieces
1 cup semisweet chocolate
1 cup finely chopped pecans
For the Cake
1 ¾ cup all-purpose flour
¼ cup unsweetened cocoa powder
2 teaspoons baking powder
½ teaspoon baking soda
½ teaspoon salt
¼ teaspoon cinnamon
8 tablespoons unsalted butter, softened
¾ cup packed light brown sugar
3 extra-large eggs
1-cup sour cream
Preheat the oven to 350 degrees. Generously grease a 9-inch Bundt pan and set aside.
Prepare the streusel: In a bowl, stir together the brown sugar, flour, cocoa powder, and ground spices. Add the butter and work it with your fingertips or two knives until it resembles pea-sized pieces. Add the chocolate and pecans.
Make the cake: in a large bowl, stir together the flour, cocoa powder, baking powder, baking soda, salt and cinnamon. In a bowl with an electric mixer, cream the butter and brown sugar on medium speed until light and fluffy. Beat in the eggs, one at a time, beating well after each addition. Scrape down the sides of the bowl with a spatula, and stir in the sour cream and vanilla until incorporated.
Add the egg mixture to the dry ingredients, stirring to combine. Do not over mix.
Scatter 1-cup streusel mixture over the bottom of the prepared pan. Top with one third of the batter, and level the top, taking care not to disturb the layer of streusel beneath. Scatter half of the remaining streusel over the batter, and top with half of the remaining batter. Level this layer of batter carefully. Make one more layer of streusel and batter and level the top. (There should be no streusel left.)
Bake the cake 45 to 50 minutes, or until a cake tester inserted in the center comes out clean. Transfer the cake to a wire rack, let stand for 5 minutes, and invert onto the rack to cool.
To store, wrap the cake in plastic wrap and store it in the refrigerator for up to 5 days. For the best flavor, return the cake to room temperature before serving.