Preheat your oven to 350°F (175°C).
In a mixing bowl, cream together the butter and sugar until light and fluffy.
Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.
In another bowl, whisk together the flour, baking powder, and cinnamon. Gradually add to the creamed mixture, mixing just until combined.
Fold in the sliced apples.
Pour the batter into a greased baking pan and spread evenly.
Bake in the preheated oven for 50 minutes, or until a toothpick inserted into the center comes out clean.
Let the cake cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.