Chocolate butter cream in between baking done by beat butter at room temperature, the salinity is noisy. Sift the cocoa, and it's down to low speed, beat together before acceleration. Put the vanilla and cream into it.
During baking chocolate buttercream. The salted butter at room temperature to soften it. Sift cocoa and icing down. Beat at low speed to combine, before accelerating to enter the vanilla and cream.
During baking, make chocolate butter cream. By beating the salted butter at room temperature to soften address. Sift cocoa and icing down. Hit with low speed well before the acceleration, put the vanilla and cream.