4.
For the frosting: Place the chocolate and butter in the top of a double boiler (or in a metal bowl nestled in a saucepan of boiling water) over medium heat, stirring regularly. When the chocolate is melted, stir in the sugar. Add the hot water, 1 tablespoon at a time, mixing until smooth. Remove from the heat and gently mix in the egg yolks, vanilla, and salt, stirring after each addition. Blend with a handheld electric mixer on low speed (or by hand) for 1 minute, or until the frosting is smooth. If the frosting is too thick to pour, add more hot water, a little at a time, to achieve a consistency that allows for drizzling.
