This is a recipe adapted from Everyday Food.
For the crumb topping: 1/4 cup (1/2 stick) cold unsalted butter, cut into pieces 1/2 cup packed light-brown sugar 1/2 cup toasted pecans, chopped 1/4 cup old-fashioned rolled oats 1/2 teaspoon cinnamon 1/4 teaspoon coarse salt For the batter: 1/2 cup (1 stick) unsalted butter, room temperature, plus more for cake pan 2 cups all-purpose flour (spooned and leveled), plus more for cake pan 1 teaspoon baking soda 1 teaspoon cinnamon 1/2 teaspoon coarse salt 1 cup granulated sugar 1/2 cup packed light-brown sugar 4 large eggs 1 cup sour cream 2 apples (about 1 pound), peeled, cored, and diced medium 1 cup toasted pecans, chopped
Preheat oven to 350 degrees. Make topping: In a medium bowl, combine butter, brown sugar, pecans, oats, cinnamon, and salt. With a pastry cutter or your hands, blend until butter is incorporated and mixture resembles coarse crumbs. Refrigerate until ready to use.
Make batter: Butter and flour an angel-food cake pan or 14-cup Bundt cake pan. In a large bowl, whisk together flour, baking soda, cinnamon, and salt. In another large bowl, using an electric mixer, beat butter and sugars on high, scraping down bowl as needed, until light and fluffy, about 4 minutes. Beat in eggs, one at a time, scraping down bowl after each addition. Add sour cream and beat to combine. With mixer on low, gradually add flour mixture and beat just until combined. With a rubber spatula, fold in apples and pecans; transfer batter to pan. Top with crumb topping and, with a butter knife, gently fold some of topping into batter. Bake until a toothpick inserted in middle comes out clean, 60 to 70 minutes. Let cool in pan on a wire rack 15 minutes, then turn out onto rack and let cool.