In a saucepan of boiling water blanch the basil for 2 seconds. Drain the basil well, pat it dry, and in a food processor blend it with the butter, the zest, and sat to taste until the mixture is smooth. Let the basil butter stand, covered tightly with plastic wrap, in a cool place for 1 hour or, chilled, overnight. Serve the lemon basil butter with steamed vegetables or grilled meats and fish. Makes about 2/3 cup.