In a large saucepan of simmering water, blanch the basil leaves for 2 minutes, then drain and squeeze out as much water as possible. In a blender, combine the basil with 1/2 cup of the olive oil and blend for 1 to 2 minutes, scraping down the sides as necessary, until very smooth. Add the remaining oil, blend just to mix and pour into a large jar. Cover tightly and let sit overnight in the refrigerator. Strain the oil through a double-thickness of slightly dampened cheesecloth into a clean glass jar. Cover tightly and use as directed in th recipe or refrigerate for up to 2 days.