Melt the chocolate slowly over low heat in a double broiler. Alternatively, you can melt the chocolate in the microwave. Start with 30 seconds, then in 10 second increments.
Put butter in a large bowl or the bowl of a stand mixer.
Sift the sugar on top of the butter.
Cream together butter and sugar. Then add vanilla and cream and beat until smooth, ca. 2 minutes.
With the mixer on low speed, add chocolate. Once fully incorporated, set the speed to high and whip the buttercream until light and fluffy, ca. 3-4 minutes.
Use to top cupcakes of frost a cake.