Preheat your oven to 350°F (175°C). Grease a 9-inch round cake pan and line the bottom with parchment paper for easy removal.
In a large mixing bowl, combine the flour, baking powder, salt, and granulated sugar. Whisk until well blended.
In another bowl, whisk together the milk, eggs, vanilla extract, and melted butter until smooth.
Pour the wet ingredients into the dry ingredients and stir until just combined. Do not overmix.
Gently fold in the sliced apples until they are evenly coated with the batter. If using, sprinkle in the cinnamon and mix lightly.
Pour the apple mixture into the prepared cake pan, spreading it evenly.
Bake in the preheated oven for 35-40 minutes, or until the cake is golden brown and a toothpick inserted in the center comes out clean.
Allow the cake to cool in the pan for about 10 minutes before transferring it to a wire rack to cool completely.
Once cooled, dust with powdered sugar if desired, and slice to serve.