Preheat oven to 175°C (350°F) and line a baking sheet with parchment paper.
In a large bowl, whisk together Greek yogurt, sweetener, egg, and vanilla until smooth.
In a separate bowl, mix oat flour, protein powder, baking soda, and salt.
Add the dry ingredients to the wet ingredients and stir until just combined.
Fold in any optional mix-ins.
Scoop dough onto the prepared baking sheet, spacing cookies slightly apart.
Gently press each cookie down, as they do not spread much during baking.
Bake for 10–12 minutes until the edges are set.
Let cookies cool on the baking sheet for 5 minutes before transferring to a rack.