01 - Preheat oven to 350°F. Grease a 9x13-inch baking dish or a 9-inch round springform pan.
02 - In a medium bowl, combine the chopped pecans, brown sugar, cinnamon, flour, and melted butter. Mix 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 together until light and fluffy. 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 butter mixture, beginning and ending with the flour. Fold 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 uniformly over the batter surface.
07 - Spread the remaining batter over the filling, then evenly distribute the rest of the pecan crunch mixture across the top.
08 - Bake for 40 to 45 minutes, or until a toothpick inserted into the center of the cake comes out clean.
09 - Allow the cake to cool in the pan for 20 minutes before slicing and serving.