Measure out two cups of the mixture. Put that into a food processor. Add 3 Tbs. of the butter, the cinnamon, and the pecans to the processor bowl. Pulse the processor until everything is barely. There should be lumps about the size of lentils or smell peas. This is the finished "streusel." Set this aside.
Grease the inside of either a 10-inch tube pan or a 12-inch cake pan.
Put what's left of the flour-sugar mixture from the first step into a mixer bowl. Add the salt and give it a quick stir with the mixer to distribute everything evenly. Add the butter and let it mix into the flour on low speed.
In another bowl (you can use the one the flour and sugars were in--you don't even have to clean it), whisk the eggs, then add the buttermilk, sour cream, and vanilla.
Pour half the egg mixture into the mixer bowl and beat on medium speed for about a minute. Scrape down the sides of the bowl, all the way to the bottom, with a rubber spatula. Start the mixer again on medium-low speed, and add the rest of the egg mixture. After about a minute, scrape down the bowl again. Now run the mixer a bit higher, until you can see the batter starting to rise and expand.
Pour about a third of the batter into the pan. Smooth it out with the rubber spatula, and sprinkle about a third of the streusel uniformly around the pan. Add another third of the batter and repeat the strewing of the streusel. Add the rest of the batter, and top it with the rest of the streusel.
Bake the cake at 350 degrees for 50-60 minutes. Check with a cake tester; when it comes out clean of interior ingredients, it's done.
Let the cake cool for about a half-hour. Remove from the pan to a rack, and cool for another hour or two. (You can serve it warm, but sometime it falls apart when you slice it.