Preheat your oven to 375°F (190°C). Lightly grease a 9-inch round springform pan. Line the bottom with parchment paper and grease the parchment paper as well. Dust with a little flour.

Peel, core, and slice the apples very thinly, about 1/16-inch thick. A mandoline works best for uniform slices, but a very sharp knife can also be used. Set aside in a large bowl.

In a medium bowl, whisk together the melted butter and granulated sugar until well combined.

Add the eggs and vanilla extract to the butter-sugar mixture, whisking until smooth and creamy. Stir in the whole milk.

In a separate small bowl, whisk together the all-purpose flour, baking powder, salt, and cinnamon (if using).

Gradually add the dry ingredients to the wet ingredients, whisking until just combined and no lumps remain. Do not overmix.

Pour the batter over the thinly sliced apples in the large bowl. Using a spatula, gently fold the apples into the batter until every slice is coated. This may seem like a lot of apples for the amount of batter, but that's what makes the cake "invisible"!

Pour the apple-batter mixture into the prepared springform pan, spreading it evenly. Gently tap the pan on the counter a few times to settle the apples and release any air bubbles.

Bake for 45-55 minutes, or until the top is golden brown and a toothpick inserted into the center (avoiding an apple slice) comes out clean. The cake will be quite moist.

Let the cake cool in the pan for 15-20 minutes before carefully removing the springform ring. Transfer the cake to a wire rack to cool completely.

Once the cake is completely cool, prepare the glaze (if desired). In a small bowl, whisk together the powdered sugar and 1 tablespoon of milk or lemon juice until smooth. Add more liquid, a few drops at a time, until you reach a pourable consistency.

Drizzle the glaze over the cooled cake. Allow the glaze to set before slicing and serving.