01 - Preheat oven to 350°F. Grease a 9x13-inch baking dish or a 9-inch round springform pan with butter or nonstick spray.
02 - In a medium bowl, combine the chopped pecans, brown sugar, cinnamon, flour, and melted butter. Stir until the mixture becomes crumbly and well distributed. Set aside.
03 - In a separate bowl, whisk together the flour, baking powder, baking soda, and salt until evenly blended.
04 - In a large mixing bowl, beat the softened butter and granulated sugar using an electric mixer until light and fluffy, about 3 minutes. Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.
05 - Alternately add the dry ingredient mixture and the sour cream to the creamed butter mixture, beginning and ending with the flour. Mix gently until just combined — avoid overmixing to keep the cake tender.
06 - Spread half of the cake batter evenly into the prepared pan. Sprinkle half of the cinnamon pecan crunch mixture over the batter in an even layer.
07 - Carefully spread the remaining batter over the pecan layer, smoothing the top. Finish by sprinkling the rest of the crunch mixture evenly across the surface.
08 - Bake for 40 to 45 minutes, or until a toothpick inserted into the center of the cake comes out clean and the top is golden brown.
09 - Allow the cake to cool in the pan for 20 minutes before slicing into squares and serving warm or at room temperature.