Preheat your oven to 175°C (350°F). Grease an 8-inch round cake pan or line it with parchment paper. Make sure the pan is prepared before mixing the batter so the cake can go into the oven promptly.
Wash, peel, and core the apples. Cut them into small, evenly sized pieces so they soften properly during baking. Choose sweet-tart apples such as Granny Smith, Gala, or Fuji. Toss the diced apples with a small pinch of cinnamon if you want extra flavor.
In a medium bowl, whisk together the all-purpose flour, baking powder, ground cinnamon, and salt. Mixing these ingredients separately helps distribute the raising agent and spices evenly throughout the cake.
In a large mixing bowl, beat the softened butter and granulated sugar until the mixture becomes pale and fluffy. This step incorporates air into the batter, helping create a light texture.
Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract until the mixture is smooth. If the mixture looks slightly curdled, do not worry; adding the dry ingredients gradually should bring it together.
Add the dry ingredients to the butter mixture in batches, alternating with the milk. Begin and end with the flour mixture. Mix gently until just combined, taking care not to overmix the batter.
Gently fold the diced apples into the batter using a spatula. If you are adding walnuts, pecans, or raisins, fold them in at this stage. The batter should be thick enough to hold the apple pieces evenly.
Combine the granulated sugar and cinnamon in a small bowl. Transfer the cake batter to the prepared pan and spread it evenly. Sprinkle the cinnamon-sugar mixture over the top. For extra richness, lightly brush the surface with melted butter before adding the topping.
Bake for 40–50 minutes, or until the top is golden brown and a toothpick inserted into the center comes out clean or with a few moist crumbs. If the top browns too quickly, loosely cover it with aluminum foil for the remaining baking time.
Allow the cake to cool in the pan for 10–15 minutes before transferring it to a wire rack. Let it cool sufficiently before slicing. Serve it plain, dusted with powdered sugar, or accompanied by whipped cream or vanilla ice cream.