In a medium saucepan, melt the butter over medium heat. Stir in the sugar, cocoa powder and salt, then gradually stir in the cream. Cook, stirring, until very hot but not boiling, about 5 minutes. Remove from the heat and stir in the vanilla. Transfer to a bowl and place in a larger bowl of ice water. Stir until cool and thick enough to spread, 5 to 8 minutes.
