Using the paddle attachment of an electric mixer, cream together the cream cheese, baking sugar replacement, and light butter until fluffy.
In a small bowl, combine cocoa powder and espresso powder. Then slowly beat the cocoa powder mixture into the creamed cheese mixture.
Add vanilla and beat until well blended. Stir in buttermilk, 1 tablespoon at a time, until frosting is a spreadable consistency. Frost cake as desired.