Preheat Oven:

Preheat your oven to 350°F (175°C). Grease and flour a 9-inch round or square baking pan, or line it with parchment paper.

Mix Dry Ingredients:

In a medium bowl, whisk together the flour, baking powder, baking soda, salt, and ground cinnamon. Set aside.

Combine Wet Ingredients:

In a large mixing bowl, whisk together the melted butter and sugar until smooth. Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.

Add Milk and Dry Ingredients:

Gradually add the dry ingredients to the wet mixture, alternating with the milk, beginning and ending with the dry ingredients. Mix until just combined. Be careful not to overmix.

Fold in Apples:

Gently fold the diced apples into the batter.

Pour into Pan:

Spread the batter evenly into the prepared baking pan.

Prepare the Topping:

In a small bowl, mix the sugar and cinnamon together. Sprinkle it evenly over the top of the cake. Drizzle the melted butter over the cinnamon sugar layer.

Bake:

Bake in the preheated oven for 35-40 minutes, or until a toothpick inserted into the center of the cake comes out clean.

Cool and Serve:

Allow the cake to cool in the pan for 10 minutes before transferring it to a wire rack. Serve warm or at room temperature.