Chocolate Glaze:
In a bowl or pan over simmering water, combine the semisweet chocolate, unsweetened chocolate and the 2 tablespoons of butter. Stir until the mixture is melted and smooth.
Remove from the heat and beat in the powdered sugar, boiling water, a pinch of salt and 1/2 teaspoon of vanilla.
Drizzle the glaze over the cooled cake.