Preheat your oven to 180°C (350°F) and line a baking tray with parchment paper.
Sift and combine flour and baking powder in a medium-sized bowl, then set it aside.
Using a larger bowl, beat eggs and sugar.
Add the melted butter, milk, and vanilla extract and continue mixing.
Combine the flour and baking powder, and mix until there are no lumps left in the apple cake batter.
Pour the batter into the lined baking sheet and use an angled spatula to even the top.
Prepare the topping by combining ground cinnamon with sugar.
Arrange the fresh apple slices on top and sprinkle them evenly with the sugared cinnamon.
Place your sheet cake into the lower part of the oven for 28 to 30 minutes or until a toothpick comes out clean when inserted into the cake.
Set aside for a few minutes to cool.
Slice your newly baked apple sheet cake into squares and serve with more cinnamon sugar on top, if desired.