Preheat your oven to 350°F (175°C). Grease and flour a 9x9-inch baking dish or line it with parchment paper.
In a large bowl, whisk together the flour, sugar, baking powder, baking soda, cinnamon, nutmeg, and salt.
In another bowl, whisk the eggs, oil, sour cream (or Greek yogurt), and vanilla extract until smooth.
Gradually add the wet ingredients to the dry ingredients, stirring until just combined.
Fold in the diced apples gently.
Spread the batter evenly into the prepared baking dish.
In a small bowl, mix the sugar, cinnamon, and melted butter. Drizzle or sprinkle this mixture over the batter.
Bake for 35–40 minutes, or until a toothpick inserted into the center comes out clean.
Let the cake cool in the pan for 10 minutes before transferring it to a wire rack. Serve warm or at room temperature.