Preheat the oven to 350°F (175°C). Grease and line an 8-inch or 9-inch round cake pan with parchment paper.
In a small bowl, combine the sliced apple, brown sugar, and cinnamon for the topping. Set aside.
In a medium bowl, whisk together the flour, baking powder, cinnamon, and salt.
In a large mixing bowl, beat the butter and sugar until light and fluffy, about 2 to 3 minutes.
Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
Add the dry ingredients to the wet ingredients in three additions, alternating with the milk. Mix just until combined.
Fold the diced apples into the batter until evenly distributed.
Spread the batter into the prepared cake pan and smooth the top.
Arrange the cinnamon sugar apple slices over the batter and lightly press them into the surface.
Bake for 40 to 50 minutes, or until a toothpick inserted into the center comes out clean.
Let the cake cool in the pan for 15 minutes before transferring it to a wire rack to cool completely.
Slice and serve as is, or enjoy with whipped cream, caramel sauce, or vanilla ice cream.