In a large bowl, mix peanut butter, protein powder, flax, maple syrup, milk, and vanilla. Stir well until combined.
Add oats and chocolate chips and mix until evenly combined.
Scoop mixture into 18 balls (using a ~2 tbsp cookie scoop) onto the parchment-lined baking sheet. Flatten each ball into a cookie shape and smooth the edges.
Enjoy immediately! Refrigerate leftovers in an airtight container.
Notes
If the dough is too sticky to handle, allow to sit for 5-10 minutes to allow the oats to absorb some moisture.