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, vanilla protein powder, baking powder, baking soda, ground cinnamon, and salt.
In a large bowl, combine the applesauce, Greek yogurt, honey, eggs, and vanilla extract. Whisk until smooth.
Gradually add the dry ingredients to the wet ingredients, stirring gently until just combined. Do not overmix.
Fold in the melted butter until evenly incorporated.
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 arranged in a single layer.
Bake for 40 to 45 minutes, or until a toothpick inserted into the center comes out clean.
Remove the cake from the oven and let it cool in the pan for 15 minutes.
Transfer the cake to a wire rack and cool completely before slicing and serving.