In a microwave-safe bowl, combine the chocolate and 1 tablespoon of the butter. Melt in 30 second increments, stirring in between until smooth. Alternatively, you can melt the chocolate and butter together in a heat-proof bowl set over a pan of barely simmering water. Set aside to cool to lukewarm.
In a large bowl, beat the remaining ½ cup butter and the cream cheese together until smooth. Beat in the powdered sweetener and the cocoa powder until well combined.
Add the chocolate and vanilla extract and beat until smooth. The mixture will be very thick at this point.
Add the cream a few tablespoons at a time until a spreadable consistency is achieved.
