1:3 ratio of basil leaves/stems, separated and a good-quality olive oil
garlic cloves, smashed (optional)
Add plenty of ice to a large steel prep bowl. Add in some cold water to create an icy bath.
Bring a salted pot of water to a boil. Throw in the basil stems and blanch for 2-3 minutes. Remove the stems from the water and place in the ice bath. Now add the leaves to the boiling water and blanch for 20 seconds. Remove from the water and add the leaves to the ice bath. Discard the boiling water.
Dry the leaves and stems very well with towels and place in a blender. Puree for one to two minutes with the olive oil, then place the puree back into the pot and heat for 3-4 minutes over medium-high heat, adding in the smashed garlic if desired. Remove from the heat and place the entire pot in the ice water bath. Stir vigorously to cool the mixture down.
Transfer the mixture to a container and place in the refrigerator for 1-2 days to allow the flavor to develop. Then strain the mixture through a fine sieve and store in an airtight container in the refrigerator. It will become butter-like in texture once chilled, but will melt very easily once exposed to room temperature. After a week, place in the freezer and use as needed in sauces and marinades or as a spreading for bread.