3/4 C sugar
6 T frozen pineapple concentrate, thawed
6 T water
4 egg yolks
3/4 lb unsalted butter, softened
1-1/3 T dark rum
Place suger, juice, and water in a heavy medium saucepan. Over high heat, stir until sugar is dissolved. Bring to a boil for 2-1/2 minutes.
Place the egg yolks in a large bowl and mix with an electric mixer on medium. Gradually add the hot syrup, pouring it over the yolks in a thin stream while mixing. Increase speed to moderately high and beat for 5 to 6 minutes, until it thickens and has the consistency of mayonnaise. Cut butter in tablespoon-sized pieces. Add butter one piece at a time until it’s all been incorporated. Reduce speed and add rum. The icing should be of spreading consistency. If it’s too soft, chill for a few minutes.