Place chocolate in top of a double boiler; bring water to a boil.
Reduce heat to low; cook until chocolate melts.
Set aside to cool.
Cream butter in a large bowl, and set aside.
Combine sugar and water in a small, heavy saucepan; cook over medium heat, stirring constantly, until mixture boils.
Boil, without stirring, until mixture reaches small thread stage (217° to 220°).
Remove from heat.
Beat egg yolks in a small bowl at high speed of an electric mixer until thick and lemon colored; gradually add hot syrup to yolks, beating until mixture cools.
Add yolk mixture, cooled chocolate, and brandy, if desired, to butter; beat at high speed of electric mixer until blended.
Cover and refrigerate until the mixture is thick enough to pipe.
Note: Butter cream may be stored in a tightly covered container in the refrigerator for up to 1 week.
Allow to stand at room temperature until mixture is softened enough to pipe.