In a medium bowl, use a whisk or fork to stir together the pecans, 2/3 cup flour, 1/2 cup brown sugar and 2 teaspoons of cinnamon.
Melt 6 tablespoons of butter in the microwave. Pour it over the pecan mixture. Stir until well combined and it looks like coarse crumbs. Put the bowl in the fridge until ready to use.
Coffee Cake
Preheat oven to 350 degrees F. Spray the bottom of a nonstick 9x13 metal or ceramic cake pan with cooking spray and set aside.
In a separate bowl, whisk together the remaining two cups of flour with one teaspoon of cinnamon and the baking powder, baking soda and salt.
In the bowl of stand mixer fitted with the paddle attachment, cream together one cup of softened butter with one cup of granulated sugar until pale and fluffy.
Add the sour cream and beat on medium speed until combined. Add the eggs and beat again. Then add the orange juice and vanilla and beat again until well combined.
Turn off the mixer and add the flour mixture. Start on low speed and then increase the speed, stopping and scraping down the bowl if needed, to mix in the flour.
Use an offset spatula to carefully spread HALF of the cake batter into the bottom of the pan.
Sprinkle HALF (about 2 cups) of the pecan cinnamon streusel evenly on top of the cake layer.
Carefully spread the remaining cake batter over the streusel layer. And then sprinkle the remaining 2 cups of streusel on top of the coffee cake.
Bake the coffee cake on the center rack of the oven for 45 - 50 minutes. Or until a toothpick inserted into the center of the cake comes out clean.
Allow the coffee cake to cool in the pan for about 20 minutes before slicing and serving.