Combine all dry ingredients (oats, protein powder, chia seeds, flaxseed, nuts, cinnamon, and salt) in a large mixing bowl. Whisk together thoroughly to prevent protein powder clumps.

Gently warm the peanut butter and maple syrup in a small saucepan over low heat or microwave for 30-45 seconds until smooth and easily stirrable. The mixture should be warm but not hot.

Pour the warm wet mixture over the dry ingredients and stir vigorously until everything is evenly coated and holds together when squeezed. If mixture seems too dry, add 1 tablespoon more peanut butter.

Fold in chocolate chips if using, then transfer mixture to a parchment-lined 8x8 inch pan. Press down very firmly with the back of a measuring cup - the firmer you press, the better they'll hold together.

Refrigerate for at least 1 hour until completely set. Run a knife under hot water between cuts for clean slices.