Preheat your oven to 350°F (175°C). Grease an 8x8-inch baking dish or line it with parchment paper.
In a large bowl, whisk together the flour, sugars, baking soda, salt, cinnamon, and nutmeg.
Add in the melted butter, sour cream, egg, and vanilla extract. Stir just until combined.
Fold in the chopped apples. Don’t overmix—this cake prefers a gentle hand.
Pour the batter into your prepared pan and spread it out evenly.
In a small bowl, combine the topping flour, brown sugar, and cinnamon. Cut in the cold butter using a pastry blender or your fingers until it forms coarse crumbs.
Sprinkle the topping evenly over the cake batter.
Bake for 35–40 minutes or until a toothpick inserted into the center comes out clean.
Let cool slightly before slicing, but I won’t blame you if you cut into it while it’s still warm.