In a medium bowl, whisk the sourdough starter, milk, egg, oil or butter, vanilla, and maple syrup until smooth.
In a separate bowl, whisk the flour, protein powder, baking powder, baking soda, cinnamon, and salt.
Fold the dry ingredients into the wet until just combined. The batter should be thick but pourable. Add 1–3 tablespoons more milk if it seems too stiff.
Heat a nonstick skillet or griddle over medium heat.
Lightly grease with butter or oil.
Scoop about 1/4 cup batter per pancake. Cook until bubbles form on top and the edges look set, about 2–3 minutes.
Flip and cook 1–2 minutes more, until golden and cooked through. Adjust heat as needed to avoid over-browning.
Serve warm with toppings like Greek yogurt, fresh fruit, nut butter, or maple syrup.