In a large mixing bowl, combine protein powder with oat flour, peanut butter, Greek yogurt and maple syrup. If using unsalted peanut butter, add a pinch of salt.
Using a spatula or a fork, stir all ingredients until combined into a smooth dough. Add more peanut butter if the dough is crumbly, or add more oat flour if sticky - this depends on protein powder used.
Line a 6 x 6 in (15 x 15 cm) pan with parchment paper or grease with little oil so that the protein bars do not stick to the pan. Transfer the dough to the prepared pan, smooth it out and refrigerate for at least 1 hour.
After the dough has set, slice into bars. Keep stored in the fridge for a maximum of 4 days.
