Preheat the oven to 350°F (175°C). Grease and flour an 8-inch round cake pan.
In a medium bowl, whisk together the flour, protein powder, baking powder, baking soda, cinnamon, nutmeg, and salt until well combined.
In a large bowl, cream the softened butter and granulated sugar together using a hand mixer or stand mixer until light and fluffy, about 3 minutes.
Add the eggs one at a time to the butter mixture, beating well after each addition.
Mix in the vanilla extract.
Alternately add the dry ingredients and Greek yogurt to the butter mixture, beginning and ending with the dry ingredients. Mix gently until just combined.
Stir in the milk until the batter is smooth.
Spread half of the batter evenly into the prepared cake pan.
Arrange half of the apple slices in a single layer over the batter.
Pour the remaining batter over the apple layer and spread evenly.
Top with the remaining apple slices, arranging them neatly on top.
Bake in the preheated oven for 40 minutes, or until a toothpick inserted into the center comes out clean.
Remove the cake from the oven and allow it to cool in the pan for 15 minutes.
Transfer the cake to a wire rack to cool completely before slicing and serving.