Preheat the oven to 350 degrees F. In a large mixing bowl, mix 2 1/2 cups flour, granulated sugar, baking powder, baking soda, and salt. Mix in the sour cream, eggs and vanilla. Then melt 8 tablespoons (1 stick) of butter and mix into the batter. Set aside.
In a medium bowl, mix the chopped pecans, brown sugar, and cinnamon together. Spread 1/3 of the batter in the bottom of a 9x13-inch baking dish. Then sprinkle 1/3 of the pecan mixture over the batter. Repeat with another 1/3 of the batter followed by the nut mixture. Then spread the last of the batter over the top.
To the remaining 1/3 of the pecan mixture, add 1/4 cup flour. Cut the last 3 tablespoons of butter into cubes and mix into the nut mixture by hand, creating small chunky pebbles of dough. Sprinkle the crumble topping over the surface of the cake.
Bake for 40-45 minutes until a toothpick inserted into the center of the cake comes out clean.
Using a toothpick or wooden skewer, poke holes over the surface of the cake for the glaze. Whisk the Kahlua and powdered sugar together in a small bowl, then drizzle over the surface while the cake is still warm. Allow the cake to cool and the glaze to harden. Cut and serve.
This cake can sit at room temperature for up to 5 days. Keep it covered.