Prep and Preheat (5 minutes): Preheat your oven to 350°F. Line a 9-inch square metal baking pan with parchment paper or grease it well with butter. You want the bottom and sides fully coated so the cake doesn't stick. Set pan aside.

Make the Dry Mix (3 minutes): In a small mixing bowl, whisk together 2 cups all-purpose flour, 1.5 teaspoons baking powder, 0.5 teaspoon baking soda, and 0.5 teaspoon salt. Whisk for about 20 seconds until everything is evenly distributed—you should see no lumps of baking soda. Set aside.

Cream Butter and Sugar (3–4 minutes at medium speed): In a large mixing bowl, combine 0.5 cup softened unsalted butter and 0.75 cup granulated sugar. Using an electric mixer on medium speed, beat for 3–4 minutes until the mixture is light, pale, and fluffy. You should be able to see air bubbles in it, and it will look almost like frosting. Checkpoint: The mixture should be noticeably lighter in color and doubled in volume.

Add Eggs (4–5 minutes): Add 2 large eggs one at a time to the butter mixture. After each egg, beat on medium speed for about 45 seconds until fully combined before adding the next one. The mixture may look slightly curdled after the first egg—this is normal. Checkpoint: After both eggs are added, the batter should be smooth and creamy.

Add Vanilla (1 minute): Add 1 teaspoon vanilla extract and mix on low speed for 30 seconds until combined.

Prepare Apple Mixture (5 minutes): While the batter rests, peel, core, and dice 3 medium Granny Smith apples into small chunks (about 0.5-inch pieces). In a small bowl, combine the diced apples with 2 tablespoons all-purpose flour, 1 tablespoon granulated sugar, 0.5 teaspoon ground cinnamon, and 0.25 teaspoon ground nutmeg. Toss gently with a spoon until apples are evenly coated. The flour will help absorb the apple juice and prevent the batter from becoming too wet. Checkpoint: All apple pieces should look slightly coated and no dry flour should be visible.

Alternate Wet and Dry (4–5 minutes): Add one-third of the dry flour mixture to the butter-egg mixture and stir on low speed for about 20 seconds until just combined. Then add half of the 0.5 cup sour cream and fold in gently by hand with a spatula, about 10 folds, until just blended. Repeat: add another third of dry mixture, fold until combined, then add the remaining sour cream and fold. Finally, add the last third of dry mixture and fold until no white streaks of flour are visible. Important: Do not overmix at this stage or the cake will be tough. Stop as soon as you see no dry flour. Checkpoint: The batter should be thick, creamy, and uniform in color with no visible lumps of flour.

Fold in Apples (2 minutes): Gently fold the spiced apple mixture into the cake batter using a spatula, making about 8–10 folds until the apples are evenly distributed throughout. Some batter may still show, but all pieces should be mixed in. Checkpoint: You should see apple chunks throughout the batter with no large clumps of apples sitting on top.

Make Crumb Topping (3–4 minutes): In a separate medium bowl, combine 0.5 cup all-purpose flour, 0.5 cup packed brown sugar, 0.25 teaspoon ground cinnamon, and 0.25 teaspoon salt. Whisk together for 15 seconds. Add 0.25 cup cold unsalted butter, cut into small cubes (about 8 pieces). Using a pastry cutter, two forks, or your fingertips, work the cold butter into the flour mixture until it resembles coarse bread crumbs or wet sand. The pieces of butter should be visible—do not fully blend. Then stir in 0.5 cup chopped pecans. Checkpoint: The topping should be crumbly with pea-sized pieces of butter visible and pecans evenly mixed throughout.

Pour Batter into Pan (2 minutes): Transfer the apple cake batter into the prepared 9-inch square pan. Spread it evenly using a spatula, making sure to fill all corners and create an even surface. The batter should reach the edges of the pan.

Add Topping (2 minutes): Sprinkle the crumb topping evenly over the batter, using your hands to gently distribute it so there are no bare spots. Do not press it down hard—let it sit loosely on top so it stays crumbly after baking.

Bake (45–50 minutes at 350°F): Place the pan on the center rack of your preheated 350°F oven. Bake for 45–50 minutes. The cake is done when the top is golden brown (the crumbs should turn a medium to dark golden color) and a toothpick or cake tester inserted into the center of the cake comes out with only a few moist crumbs attached—not wet batter, but not completely dry. If the toothpick comes out with wet batter, bake another 2–3 minutes and check again. Checkpoint: The cake should be golden brown, smell strongly of cinnamon and apples, and the crumb topping should be set and no longer loose.

Cool in Pan (15 minutes): Remove the pan from the oven and place it on a wire cooling rack or a folded kitchen towel on the counter. Let the cake cool in the pan for 15 minutes. This allows it to set and firm up so it won't fall apart when you cut it. During this time, the cake will continue to cook slightly from residual heat.

Final Cool and Serve (Optional, up to 2 hours): You can serve the cake warm after 15 minutes in the pan, or let it cool completely at room temperature (about 1 hour total) for a firmer texture. Cut into 9 squares if using a 9-inch square pan, or into your desired serving size.