Preheat the oven to 350°F. Spray an 8 X 8-inch baking dish with baking spray.
Combine flour, pecans, brown sugar, cocoa powder, cinnamon, and salt for crumb topping in a medium bowl. Add melted butter. Stir until evenly moistened. Stir in chocolate chips. Set aside.
1 1/2 cups (212g) all-purpose flour, 3/4 cup chopped pecans, 3/4 cup (150g) light brown sugar, 3 Tablespoons cocoa powder, 1/2 teaspoon ground cinnamon, Pinch of salt, 1/2 cup salted butter, 1 cup semisweet chocolate chips
Whisk flour, baking powder, baking soda, and salt in a separate bowl. Set aside.
2 cups (282g) all-purpose flour, 1 teaspoon baking powder, 1/2 teaspoon baking soda, 1/2 teaspoon salt
In a large mixing bowl, cream butter and sugar together for 1-2 minutes until light and fluffy. Add eggs, vanilla, and sour cream. Beat again, scraping bottom and sides of bowl until combined.
1/2 cup salted butter, 1 cup (200g) granulated sugar, 2 large eggs, 2 teaspoons vanilla extract, 1 cup sour cream
Add the flour mixture, mixing on medium speed just until combined.
Transfer cake batter to the prepared baking dish. Sprinkle the crumb topping evenly over cake batter.
Bake for 50-60 minutes until done. A wooden skewer inserted into the center of the cake should come out clean with just a few crumbs attached.