The sauce of choice in Liguria, along the Italian Riviera. Fresh basil leaves are really a must; the small, very tender ones are optimum. The brightness of the taste of pesto, when done fresh and well, can not really be adequately described.
for the pesto sauce:
Using a blender (pesto was traditionally made in a mortar with pestle, but a blender works excellently), put in the basil and garlic and mix for a few seconds. Then add the pine nuts and olive oil and salt, and blend at a high speed until the sauce approaches being creamy (not too much, however, as a little roughness in the texture is wonderful).
When the blending is completed, transfer the sauce into a mixing bowl and stir in the two grated cheeses, and then the softened butter, making sure you get a thorough and uniform melding.