Tear the basil leaves from the stem and discard of the stem.
Bring a pot of water to the boil. Once boiling, turn off the heat and blanch the basil leaves in the hot water for 10 seconds to set the colour. Do not leave them in the water for much longer than this or they will brown. Using a slotted spoon, transfer the blanched basil leaves into a bowl of icy cold water. Leave to sit for several minutes or until cool.
Using your hands, remove the basil leaves from the icy cold water and squeeze out as much water as possible.
Place the basil leaves into a food processor with the oil and blend on high until well combined.
Place a sieve over a bowl and line with cheesecloth or a chux. Pour the basil into the cheesecloth and leave to naturally strain. This can take up to 20 minutes. Give the cloth a squeeze to remove as much of the oil as possible. Transfer the oil to a sterilised jar and leave on your counter bench for up to 2 weeks.
Prepare the salad by placing the rockmelon onto a large platter. Scatter the prosciutto over the rockmelon. Tear up the mozzarella and place over the rockmelon and prosciutto. Season with salt and pepper and drizzle 3tbsp of the basil oil all over. Serve immediately.