Using a hand mixer or an electric mixer fitted with the paddle attachment, cream the stick of butter in a large bowl until soft, about 1 minute. With the mixer on low, gradually add the sugar. When all the sugar has been incorporated, add the vanilla and salt. Increase mixer speed to high and cream the ingredients until light and fluffy, about 4 minutes.
Melt the chocolate and the remaining butter in a double boiler over gently simmering water. Beat into the buttercream mixture until well combined.