Preheat oven to 350F and grease a 9x13 baking dish, so the cake releases cleanly after baking. Spread a light, even coating of grease across the bottom and sides.

Make crumb topping by combining all-purpose flour, brown sugar, cinnamon, and salt, then drizzle in melted butter and toss with a fork until large crumbs form. Refrigerate the crumb mixture until chilled and clumpy, about 10 minutes.

Whisk together all-purpose flour, baking powder, baking soda, cinnamon, nutmeg, and salt until evenly combined with no visible streaks. Use the dry mix as the foundation for the cake’s lift and spice.

Beat softened unsalted butter and granulated sugar until fluffy, then add eggs and vanilla and mix until smooth. The batter should look lighter in color and slightly thicker.

Beat in sour cream until incorporated, keeping the mixture cohesive. Scrape down the bowl so no sour cream pockets remain.

Fold the flour mixture into the wet ingredients until just combined. Stop as soon as no dry flour shows for a tender crumb.

Fold in diced apples so they’re evenly distributed throughout the batter. The apples should be visible in the batter rather than clumped.

Spread the batter in the prepared 9x13 baking dish and cover evenly and generously with the chilled crumb topping. Press lightly so the crumbs adhere on top.

Bake at 350F for 45-50 minutes until a toothpick comes out clean. Look for a golden crumb surface and set edges.

Dust with powdered sugar before serving for a light, sweet finish. Slice while slightly warm or at room temperature.