Preheat the oven to 350°F (175°C) and grease a 9-inch round cake pan.
In a large mixing bowl, combine the flour, sugar, baking powder, salt, and ground cinnamon. Mix well to ensure the dry ingredients are evenly distributed.
In a separate bowl, whisk together the melted butter, eggs, and vanilla extract until well combined.
Gradually add the wet ingredients to the dry ingredients, stirring gently with a spatula until just combined. Be careful not to overmix, as this can result in a dense cake.
Gently fold in the sliced apples until they are evenly distributed throughout the batter.
Pour the batter into the prepared cake pan, spreading it out evenly with the spatula.
Bake in the preheated oven for about 40-45 minutes, or until the cake is golden brown and a toothpick inserted into the center comes out clean.
Remove the cake from the oven and let it cool in the pan for about 10 minutes. Then, transfer it to a wire rack to cool completely.