01 - Preheat oven to 350°F. Grease and flour a 9x13-inch baking pan, ensuring even coverage on all surfaces.
02 - In a medium bowl, combine the chopped pecans, brown sugar, flour, cinnamon, and melted butter. Stir until the mixture resembles coarse crumbs and set aside.
03 - In a medium bowl, whisk together the flour, baking powder, baking soda, and salt until evenly distributed.
04 - In a large bowl, beat the softened butter and granulated sugar together using an electric mixer on medium speed until pale, light, and fluffy, about 3 minutes.
05 - Add the eggs one at a time to the butter mixture, beating well after each addition. Blend in the sour cream and vanilla extract until fully incorporated.
06 - Gradually fold the dry ingredient mixture into the wet mixture using a spatula, stirring just until no dry streaks remain. Do not overmix.
07 - Spread half of the batter evenly across the bottom of the prepared pan. Sprinkle half of the pecan topping over the batter layer. Spread the remaining batter on top and finish with the rest of the topping.
08 - Bake for 35 to 40 minutes, or until the top is golden brown and a toothpick inserted into the center emerges clean.
09 - Let the cake cool in the pan on a wire rack for at least 20 minutes before slicing into squares and serving.