Position rack in center of oven and heat oven to 350 degrees (325 if using dark, nonstick pan). Generously butter 10" tube pan with a removable bottom.
Topping: In a 2 quart saucepan, heat butter over medium heat until almost melted. Remove from heat and cool to tepid. In a medium bowl, combine flour, pecans, granulated sugar and light brown sugar, cinnamon, baking powder, and salt and stir with a fork. Add the flour mixture to the butter and stir until evenly moistened and crumbly.
Filling: In a food processor, pulse the pecans, sugars, cinnamon, and cocoa 4-6 times to combine and chop the pecans.
Cake: In a medium bowl, whisk flour, baking powder, baking soda, and salt. In the bowl of a stand mixer fitted with paddle attachment, beat the butter on medium speed until smooth and creamy, 1-2 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 down and add 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, beginning and ending with the flour and scraping the bowl as needed.
Spoon 2 generous cups of the batter into prepared pan. Smooth with the back of a soup spoon, 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 fillinf with about 2 cups of the batter, dropping dollups around the pan and smoothing with the spoon to sides of the pan and then to the center. Don't lift the spoon while smoothing or you will disturb the filling. Sprinkle another 1/2 cup of filling over the batter and cover with 2 more cups of batter. Layer on the remaining filling and then the remaining batter for four layers of batter and three layers of filling.
Insert a table knife 1" 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 the knife blade, and spacing the circles about 1" apart. This distributes the filling but still keeps it clearly defined. Smooth the top of the batter with the back of the soup spoon.
Take a handful of the streusel 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 batter.
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 in to the center of the cake comes out clean, 70-75 minutes. Transfer to a wire rack and let cool for at least an hour before removing from pan.
Gently lift up the center tube while carefully pushing the bottom upward with your opposite hand. If the cake doesn't release easily, run a thin knife between the cake and the tube and between the cake and the outer wall of the pan. Once the cake is out, run the knife under the bottom, too.
Fold a 12" square of aluminum foil into quarters. Measure 1 1/4" from the inside tip of the foil and cut a arc to make a hole for the tube when you unfold the foil. Unfold and place over the cake, pressing gently onto the top of the cake and molding the foil around its sides to hold in all the streusel.
Invert the cake onto a plate and remove the center tube. Invert the cake onto a rack, remove the foil, and cool right side up.