Preheat the oven to 350 degrees, and set an oven rack in the lowest level of the oven. Grease the inside of a standard bundt cake pan.
Make the streusel by pulsing the flour, sugar, cinnamon, and salt in a food processor to combine. Add the butter and pecans to the food processor bowl and pulse the mixture 5 times until the butter has been cut up into pea sized pieces. Keep this mixture in the fridge for later.
In a small mixing bowl, whisk to combine the eggs, 1 cup of sour cream, and vanilla.
In the bowl of a stand mixer fitted with the paddle attachment, add the flour, sugar, baking powder, baking soda, and salt, and mix on low speed for 30 second to combine.
Add the butter and other 1/2 cup sour cream and mix on low until combined. Once the flour has been moistened, increase the speed to medium and beat for 30 seconds. Scrape the sides of the bowl, then turn the mixer to low and add the egg mixture in 3 installments, beating for 20 seconds on an increased speed of medium and scraping the bowl in between each set.
For the final mixing, increase the speed to medium high and beat the mixture for 60 seconds until the mixture looks more voluminous and has become a bit pale.
Sprinkle a handful or two of the nut streusel into the bottom (which is the top) of the bundt pan, then add a few cups of the batter. Swirl with a knife or spatula. Keep repeating this process of adding streusel and batter, and swirling, until all is used up. Bake the cake on the bottom shelf of the oven for about 60 minutes, until a toothpick inserted into the center of the cake comes out clean. Let the cake cool for at least 1 hour before attempting to turn it out. FYI, this cake will taste better the next day (more moist and cohesive in flavor and texture). Enjoy!