Position a rack in the center of the oven and heat the oven to 350°F (325°F if using a dark nonstick pan). Generously butter a 10-inch tube pan with a removable bottom.
Make the topping: In a 2-quart saucepan, heat the butter over medium heat until almost melted. Remove from the heat and cool to tepid. In a medium bowl, combine the flour, pecans, both sugars, cinnamon, baking powder, and salt and stir with a fork. Add the flour mixture to the butter and stir until evenly moistened and crumbly.
Make the filling: In a food processor, pulse the pecans, both sugars, cinnamon, and cocoa 4 to 6 times to combine and chop the pecans.
Make the cake: In a medium bowl, whisk the flour, baking powder, baking soda, and salt. In the bowl of a stand mixer fitted with the paddle attachment, beat the butter on medium speed until smooth and creamy, 1 to 2 minutes. Add the sugar slowly, beating until combined. Scrape the bowl. Beat in the eggs one at a time, blending each one completely before adding the next. Scrape the bowl and blend in the vanilla. On low speed, alternate adding the dry ingredients and the sour cream, adding the flour in four parts and the sour cream in three parts, beginning and ending with the flour, and scraping the bowl as needed.
Layer and marble the batter and filling: Spoon 2 generous cups of the batter into the prepared pan. Smooth with the back of a soupspoon, spreading the batter to the side of the pan first and then to the center. Sprinkle about 1/2 cup of the filling evenly over the batter. Cover the filling with about 2 cups of batter, dropping dollops around the pan and smoothing with the spoon. Sprinkle another 1/2 cup filling evenly over the batter and cover with 2 more cups batter. Layer on the remaining filling and then the remaining batter. (You'll have four layers of batter and three layers of filling.) Insert a table knife 1 inch from the side of the pan straight into the batter going almost to the bottom. Run the knife around the pan two times, without lifting up the blade, spacing the circles about 1 inch apart. Smooth the top with the back of the soupspoon.
Top and bake the cake: Take a handful of the streusel crumbs and squeeze firmly to form a large mass. Break up the mass into smaller clumps, distributing the streusel evenly over the batter. Repeat with the remaining streusel. Press the streusel lightly into the surface of the cake. Bake until the top of the cake is golden brown, the sides are beginning to pull away from the pan, and a wooden skewer inserted into the center of the cake comes out clean, 70 to 75 minutes. Transfer to a wire rack and let cool for at least an hour before removing from the pan. Check out our tips on turning out the cake.
This cake keeps at room temperature, well wrapped or under a cake dome, for up to 5 days; you can freeze it for up to 3 months.