In a small bowl, whisk together the egg yolk, Greek yogurt, vanilla extract, oat flour, and vanilla protein powder until a smooth, thick paste forms.
In a separate clean glass or metal bowl, combine the egg whites and cream of tartar.
Using a hand mixer, beat the egg whites on medium-high speed until stiff, glossy peaks form that stand straight up when the beaters are lifted.
Gently fold one-third of the whipped egg whites into the yolk mixture to lighten it, using a silicone spatula to avoid deflating the air.
Carefully fold the remaining egg whites into the batter in two batches until just combined and no white streaks remain.
Heat a non-stick skillet over low heat and lightly coat with avocado oil.
Scoop the batter into three tall mounds in the skillet, add a teaspoon of water to the empty spaces of the pan, and cover with a lid to steam for 4-5 minutes.
Carefully flip each pancake, add another teaspoon of water, cover, and cook for an additional 3-4 minutes until golden and set.
Plate the pancakes immediately and top with fresh mixed berries and a drizzle of maple syrup.