Preheat the oven to 350°F (175°C). Grease and flour a 9-inch round cake pan.

In a mixing bowl, whisk together the flour, sugar, baking powder, cinnamon, and salt.

Add the softened butter, eggs, and vanilla extract to the dry ingredients. Mix until well combined.

Fold in the diced apples until evenly distributed.

Pour the batter into the prepared cake pan.

In a small bowl, mix together the topping ingredients (cinnamon and sugar) and sprinkle over the cake batter.

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

Allow the cake to cool in the pan for 10 minutes before transferring to a wire rack to cool completely.

Slice and serve the cake warm or at room temperature.