Makes one 7" layer cake (8 servings).
A note about substitutions: because each type of protein powder has a distinct flavor and texture when baked, I would not try substituting different types of protein powders in the cake recipe. Casein may be substituted for the whey protein in the frosting, however.
Almond meal can be made by pulsing almonds in a food processor. Sift out the large pieces and pulse again. Be careful not to overprocess, as this will give you almond butter.
Also note, because apples oxidize, the frosting will slightly discolor if left for more than a few hours. If you need to prepare your cake several hours in advance, I would suggest using agave (1 ½ tablespoons) or another natural sweetener instead of the apples in the frosting. (The apples in the cake will be fine.)