chocolate buttercream

1-1/2 oz unsweetened chocolate
2/3 c sugar
1/3 c water
2 egg yolks
1/2 c butter, soft
1 T rum (optional)

Melt chocolate in glass bowl over simmering water. Set aside to cool. Combine sugar and water in a heavy saucepan, bring to boiling, cook till temperature reaches 240 degrees (use a candy thermometer). In a small mixing bowl, beat egg yolks till thick and lemon colored. Very gradually add the hot sugar syrup to the egg yolks, beating constantly; continue beating until mixture is completely cooled. Beat in butter, 1 T at a time. Beat in melted chocolate and rum, beating until buttercream is thick.