1. Place the chocolate and butter in a 4-6 cup heatproof bowl. Place bowl in a wide skillet of barely simmering water, over low heat. Stir frequently until chocolate and butter are completely melted and smooth. Remove bowl and set aside. Leave skillet on low heat.
2. Place egg yolk in a small bowl. Gradually wisk in 1/4 cup boiling water. Place bowl in skillet and stir constantly with a spoon until yolk mixture thickens slightly to the consistency of half and half (it will be between 160 and 165 degrees and safe from salmonella) Remove yolk mixture from hot water and scrape immediately into melted chocolate. Stir gently just until chocolate and butter are completely melted and mixture is smooth. Pour through a fine strainer into a clean bowl. Cover and chill until firm, two hours or more.
3. To form truffles: Remove truffle mixture from refrigerator and allow to soften about 30 minutes (if mixture is very hard) Pour cocoa into pie plate. Dip a mellon baller or small spoon into a glass of hot water; wipe off excess water and scrape across surface of chilled mixture to form a rough 1 inch ball. Pinch the truffle into shape with your fingers if necessary, it should not be perfectly round. Deposit truffle into cocoa. Repeat with remaining truffle mixture.
4. Gently shake pie plate to coat truffles with cocoa. Store truffles tightly covered, and refrigerate up to two weeks, or freeze up to 3 months. Makes about 30 bite-size truffles.