In a large ceramic or stainless steel dish, mix together the Chermoula, 80ml of olive oil and juice of one lemon to form a thin paste. Add the prawns and use your hands to mix, making sure that they are all thoroughly coated. Allow to marinate for at least 4 hours or, ideally, overnight.
To make aioli, whisk together the egg yolks, mustard and minced garlic cloves with a good pinch of salt. Gradually add the extra virgin olive oil and then the vegetable oil in a thin, steady stream, until the mix has emulsified and has reached your desired texture. Mix through the lemon juice and season with salt and pepper to taste. Set aside.
Heat a barbecue to high and grill the prawns for 3-4 minutes each side, until just cooked. Drizzle with any remaining marinade whilst cooking.