8 tablespoons (1 stick)
unsalted butter, softened, plus more for greasing
¾ cup
granulated sugar
1
large egg, room temperature
1 teaspoon
pure vanilla extract
½ cup
full fat sour cream
⅓ cup
whole milk
1
tart apple, such as a Granny Smith, chopped into ½-inch pieces (about 1 cup)
Method
Toast the pecans. In a small skillet over medium heat, toast the pecans, stirring often, until fragrant and lightly golden, about 5 minutes. Transfer to a plate to cool.
Preheat the oven to 350°F with a rack in the center position.
Make the crumb topping. In the base of a food processor, combine the flour, brown sugar, granulated sugar, cinnamon, salt, and butter. Pulse until combined and the mixture resembles coarse sand. Transfer to a medium bowl and use your hands to squeeze the mixture together to form large clusters. Refrigerate until ready to use.
Make the cake batter. In a medium bowl, whisk together the flour, baking powder, baking soda, salt, and cinnamon. In a stand mixer or large bowl, cream together the softened butter and sugar until light and fluffy, about 3 minutes. Add the egg and vanilla and mix until combined, about 1 minute. Add half of the flour mixture and mix until incorporated. Add the sour cream and milk and mix until combined, then add the remaining flour mixture. Mix until just combined. Do not overmix.
Grease a 9 x 9-inch baking dish with butter and dust with flour.
Break up the chilled crumb topping with your hands, keeping some dime-size pieces intact. Spread half of the batter in the prepared pan. Top with the chopped apples, gently pressing them into the batter to spread it to the edges. Sprinkle with one-third of the crumb topping. Add the remaining batter, then top with the toasted pecans and remaining crumb topping. Gently press the toppings to ensure the batter reaches the edges.
Bake for 45 to 50 minutes, or until a toothpick inserted in the center comes out clean. Cool on a wire rack for 30 minutes before serving.