Weigh the oats, peanut butter, honey, protein powder and flax when possible. Bring stiff peanut butter to a workable room temperature.
Stir peanut butter, honey and vanilla in a medium bowl until glossy and completely uniform.
Fold in oats, protein powder, ground flax, cinnamon and salt until no dry streaks remain.
Rest the mixture at room temperature for 10 minutes so the oats and flax can absorb moisture.
Firmly squeeze 1 tablespoon. If it crumbles, add milk 1 teaspoon at a time, up to 3 tablespoons. If it smears, add oats 1 tablespoon at a time and wait 2 minutes.
Fold in optional mini chocolate chips only after the base mixture passes the squeeze test.
Divide with a 1 1/2-tablespoon scoop into 20 even portions on a parchment-lined tray.
Compress each portion once, then roll briefly between the palms until round. Chill portions 10 minutes first if they become sticky.
Refrigerate the tray for 30 minutes, then transfer the balls to a covered refrigerator container.