Add the basil, lemon juice, pumpkin seeds, tahini, garlic, nutritional yeast, hemp seeds and olive oil to a food processor and pulse until roughly combined. Season with salt and pepper. With the processor running, add the cold water a little at a time, checking the consistency as you go - you may not need the full 50ml. Keep pulsing until it's a coarse, spoonable pesto, or longer if you prefer it smoother and less grainy.