Add the chopped chocolate to the top of a double boiler set over barely simmering water. Stir until melted and smooth. Set aside to cool, but still pourable.
While the chocolate is melting, combine the coffee, water and cocoa in a small saucepan. Heat on low and whisk until smooth and slightly thickened. Remove and set aside to cool to room temperature.
Add the butter and 1 cup of the confectioners' sugar to a large mixing bowl. Beat until the butter is light and fluffy. Add the cooled, melted chocolate, vanilla and salt. Blend on medium until smooth. Add the cooled cocoa mixture and beat until incorporated. Gradually add the remaining 3 cups of confectioners' sugar, blending until smooth.
Frost cake or cupcakes as desired
Adapted from a recipe published in Bon Appetit Magazine