Place butter in bowl of a mixer and beat on medium for 1-2 minutes. Butter will be pale in color and should have a nice light texture.
Carefully add in powdered sugar and beat for 4-5 more minutes until it looks nice and fluffy.
Add in cocoa powder and salt, mixing until fully incorporated into the buttercream.
Add vanilla and the melted/cooled chocolate. Beat until the buttercream is a lovely even brown color and the texture looks rich and fluffy, about a minute.
Notes
This recipe makes enough buttercream to frost an 8 inch double layer cake.