Preheat oven to 350° F and prepare pans. Grease or line a 9x9 inch pan with parchment paper.
In a small mixing bowl, stir together the brown sugar, cinnamon, and pecans (if using). Set aside.
With an electric mixer, cream the butter and sugar at medium speed in a large mixing bowl about 2-3 minutes until light and fluffy. Add the eggs, sour cream, and vanilla and mix until the thoroughly combined and smooth.
In a separate bowl, add all dry ingredients and mix until combined. On low speed, add the dry ingredients into the large bowl of wet ingredients and mix just until incorporated. Do not overmix or it will toughen the cake. The batter will be fairly thick.
Spread half the cake batter into the pan. Top evenly with about ⅔ of the cinnamon brown sugar mixture. Carefully spread the remaining cake batter and gently spread over the filling. Sprinkle the top with the remaining cinnamon sugar mixture.
Bake the coffee cake in preheated oven for about 35-40 minutes, or until center is firm and a tester comes out clean.