Preheat oven to 350°F. Grease a 9x9 square pan and line with parchment paper, if desired. Set aside.
In a stand mixer or large bowl, cream together the softened butter and sugars until light and fluffy, about 2 minutes. Scrape down the bowl. Add in the sour cream, eggs, and vanilla extract.
To the bowl, add the flour, baking soda, salt, and cinnamon. Start mixing. Once almost completely combined, slowly pour in the buttermilk. Continue mixing until everything is smooth and incorporated.
Make the cinnamon center mixture. In a small bowl, mix together the brown sugar and cinnamon.
Pour about half of the coffee cake batter into the prepared pan, and spread in an even layer. Sprinkle the cinnamon mixture all over. Pour the other half of the coffee cake batter on top, and carefully spread it into an even layer.
Now, make the streusel topping by combining all streusel ingredients in a separate bowl. It should become a thick, crumbly streusel mixture. Sprinkle evenly on top of the batter.
Bake for 55-60 minutes. Allow to cool completely before slicing. Enjoy!