I use this recipe from River Cafe Green:-
2 handfuls fresh basil - washed & dried
2 small garlic cloves - peeled
10g pinenuts
50g parmesan
60ml olive oil
1/4 to 1/2 tsp salt
Crush the salt and some of the basil leaves in a pestle & mortar, add the garlic and more basil leaves, a few at a time. Add the pinenuts and any remaining basil and crush until smooth. Put the sauce into a bowl, fold in the cheese, pour in the olive oil, slowly, stirring and mixing until the sauce has a creamy consistency.
The original recipe calls for a whole teaspoon of salt, which I find a bit excessive. Give it a try and amend the quantities to suit your own taste.