Preheat oven to 325°F and line a baking sheet with parchment paper. In a bowl, mix together the oats, baking powder, cinnamon, and salt. Add the diced apple, melted butter, almond butter, liquid egg, vanilla, and maple syrup. Stir until everything is well combined. Chill the dough for 30 minutes. Divide the dough into 4 equal portions and roll each into a ball. Place on the baking sheet with plenty of space between them. Gently flatten each cookie into a thick, round disk, about ¾–1 inch thick. Keep them fairly compact so they bake up thick rather than spreading too much. Bake at 325°F for 15–18 minutes, until the edges are set but the centers still look slightly soft. Let the cookies cool on the baking sheet for 10–15 minutes, then transfer to a cooling rack and allow them to cool completely. While the cookies cool, whisk together the Greek yogurt, maple syrup, vanilla, and salt. For a thicker frosting, whisk in 1–2 tbsp protein powder or collagen powder. Refrigerate the frosting for 15–20 minutes to help it firm up. Once the cookies are completely cool, spread the maple Greek yogurt frosting generously over each cookie and enjoy!
