Prep oven and pans. Position rack to center of oven and preheat oven to 350°F (180°C). Grease a 9x9 (or 8x8) pan with butter or non-stick cooking spray.
Make topping. In a small mixing bowl, stir together brown sugar, cinnamon, and nuts. Set aside. ½ cup (107 g) light brown sugar 2 teaspoons ground cinnamon ½ cup (57 g) chopped pecans
Combine dry ingredients. In a small mixing bowl, whisk together flour, baking powder, baking soda, and salt. Set aside. 2 cups (240 g) all-purpose flour 1 teaspoon baking powder ¼ teaspoon baking soda ½ teaspoon salt
Cream butter and sugars. In a large mixing bowl, with an electric mixer cream butter and sugars together until light and fluffy, about 2 minutes. 1/2 cup (1 stick, 113 g) unsalted butter, ½ cup (100 g) granulated sugar ½ cup (107 g) packed light brown sugar
Add wet ingredients. Add eggs, one at a time, mixing until incorporated. Scrape down the bowl as needed. Add vanilla and mix to combine. 2 large eggs 2 teaspoons vanilla extract
Alternate dry with sour cream. Add the flour mixture, alternately with the sour cream in 3 additions, starting and ending with the flour. 1 cup (227 g) sour cream
Layer the batter with the topping. Spread half the batter into the pan. Top with half of the brown sugar topping. Spread remaining batter on top and sprinkle with remaining topping.
Bake. Bake for 30-40 minutes or until golden brown, and the inserted toothpick comes out clean. Remove from the oven and let cool before serving.