Preheat the oven to 350 degrees F. Line two baking sheets with parchment paper.
Start by preparing the batter for the whoopie pie cakes. In a mixing bowl, combine the flour, protein powder, cocoa powder, baking soda, and salt.
In another large mixing bowl, combine the allulose, avocado oil, yogurt, milk, egg, and vanilla. Whisk until well combined.
Pour the dry ingredients into the wet ingredients, and stir until just combined.
Portion out the batter onto the parchment lined baking sheets, about 1 ½ tablespoons each. Bake at 350 degrees F for 10 minutes. Let cool after baking.
While the cakes are cooling, prepare the filling. In the bowl of a stand mixer (or using a bowl with a hand mixer), combine the Greek yogurt, peanut butter, maple syrup, and vanilla. Using the whisk attachment on the mixer, combine the ingredients until light and fluffy.
Portion out the filling onto 10 cakes, and top with the other 10 cakes. Enjoy!