Step 1: Prepare the Oven and Pan
Set the oven temperature to 175°C (350°F). An 8 or 9-inch square or springform baking pan should be greased and floured.
Step 2: Make the Crumble Topping
Mix the flour, both sugars, cinnamon, and nutmeg in a medium-sized bowl. Incorporate the chilled, cubed butter. Cut the butter into the dry ingredients with your fingertips or a pastry cutter until the mixture looks like coarse crumbs. While you prepare the cake batter, keep the bowl refrigerated.
Step 3: Prepare the Apples
Peel, core, and dice your apples. Set them aside.
Step 4: Make the Cake Batter (Dry Ingredients)
Mix the flour, baking powder, salt, and cinnamon in a different medium basin. Put aside.
Step 5: Make the Cake Batter (Wet Ingredients)
Beat the granulated sugar and room-temperature butter in a large basin with an electric mixer until light and fluffy, about 2 to 3 minutes.
Step 6: Combine Wet and Dry
Beat well after adding each egg one at a time. Add the vanilla essence and stir. Next, alternately add the flour mixture and buttermilk, beginning and finishing with the flour mixture. Do not overmix; simply blend the ingredients. Some bumps are OK.
Step 7: Fold in Apples
Using a spatula, gently incorporate the chopped apples into the cake batter.
Step 8: Assemble the Cake
Evenly distribute the batter after pouring it into the prepared pan.
Step 9: Add the Topping
Evenly distribute the cooled crumble topping on top of the cake mixture.
Step 10: Bake
A toothpick placed in the center of the cake should come out clean after 55 to 65 minutes of baking. For the final 10 to 15 minutes of baking, you may lightly cover the pan with foil if the topping starts to brown too rapidly.
Step 11: Cool and Serve
Before slicing and serving, allow the cake to cool in the pan for at least 15 to 20 minutes.