Preheat oven to 350°F (175°C) and line a 12-cup muffin tin with paper liners.

In a large mixing bowl, whisk together the oat flour, whey protein powder, baking powder, and cinnamon.

In a separate medium bowl, beat the eggs and whisk in the applesauce and almond milk until well combined.

Pour the wet ingredients into the dry ingredients and stir until just combined; do not overmix.

Gently fold the diced apples into the batter.

Distribute the batter evenly across the 12 muffin cups.

Bake for 18 to 22 minutes, or until a toothpick inserted into the center comes out clean.

Remove from the oven and transfer cupcakes to a wire rack to cool completely.

In a small bowl, prepare the frosting by whisking together the peanut butter powder, casein protein powder, Greek yogurt, and maple syrup until smooth.

Once the cupcakes are completely cool, frost each one evenly using a piping bag or spatula.