Preheat Oven:
Preheat your oven to 350°F (175°C). Grease and flour a 9-inch baking pan or line with parchment paper.
Mix Dry Ingredients:
In a bowl, whisk together flour, baking powder, baking soda, salt, cinnamon, and nutmeg.
Cream Butter and Sugar:
In a large mixing bowl, beat the butter, granulated sugar, and brown sugar until light and fluffy (about 3 minutes).
Add Eggs and Vanilla:
Add eggs one at a time, then stir in vanilla extract.
Combine Mixtures:
Gradually add the dry mixture to the butter mixture, alternating with milk or sour cream. Mix until just combined.
Fold in Apples:
Gently fold in diced apples. The batter will be thick — that’s what keeps the cake moist.
Optional Crumb Topping:
For an Apple Coffee Cake, combine the crumb topping ingredients and sprinkle over the batter.
Bake:
Pour batter into the prepared pan. Bake for 40–45 minutes, or until a toothpick inserted in the center comes out clean.
Cool and Serve:
Let the cake cool in the pan for 10 minutes, then transfer to a rack. Dust with powdered sugar or drizzle with caramel if desired.