Gently wash basil leaves (the stems should be discarded) and dry on paper towels. Don't rub them. Set aside.
To make the pesto, pound the garlic with salt in a mortar and pestle until smooth. Add pine nuts and keep pounding until crushed and creamy. Add the basil leaves (in batches) and start crushing them by pressing and rotating the pestle against the walls of the mortar. Pound, until you have a thick green paste.
Stir in Parmesan cheese. Gradually add the olive oil to the mixture as you pound. Mix everything well till the olive oil is evenly distributed.
Use immediately or store in an airtight container in the refrigerator for up to 1 week.