01 - Preheat oven to 350°F. Grease and flour a 9x13-inch baking pan, ensuring even coverage on all sides and corners.
02 - In a medium bowl, combine the chopped pecans, packed light brown sugar, flour, ground cinnamon, and melted butter. Stir until the mixture resembles coarse crumbs and set aside.
03 - In a bowl, whisk together the all-purpose 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 until pale, light, and fluffy, approximately 3 minutes.
05 - Add the eggs one at a time to the creamed mixture, beating well after each addition. Blend in the sour cream and vanilla extract until smooth.
06 - Gradually fold the dry ingredient mixture into the wet ingredients using a spatula, mixing until just combined. Avoid overmixing to keep the cake tender.
07 - Spread half of the batter evenly across the bottom of the prepared pan. Sprinkle half of the streusel topping over the batter layer. Spread the remaining batter on top and finish with the rest of the streusel.
08 - Bake for 35 to 40 minutes, or until a toothpick inserted into the center comes out clean and the topping is golden brown.
09 - Allow the cake to cool in the pan on a wire rack for at least 20 minutes before slicing into squares and serving.