In a large bowl with an electric mixer beat egg yolks until thick and pale.
In a small saucepan heat sugar and water over moderately high heat, stirring, until a candy thermometer registers 215°F. Remove pan from heat. With mixer on low-speed, add sugar mixture to egg yolks in a slow stream. Increase speed to high and beat the mixture until thick and ribbon-like. Whisk in buttermilk.
Freeze in an ice-cream maker following manufacture’s directions.
Makes about 1 pint
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