In the bowl of a stand mixer fitted with the paddle attachment, beat the butter on medium-high speed for 6-7 minutes. Scrape down the sides of the bowl with a rubber spatula.

In a large bowl, sift together the powdered sugar and cocoa powder.

With the mixer on low speed, slowly add the powdered sugar and cocoa and continue beating until it is fully incorporated, stopping to scrape down the sides of the bowl as needed.

Add in salt and vanilla and mix to combine. Spoon in the melted and cooled chocolate and mix on low speed until no streaks remain. Add in 3 tablespoons of heavy cream or milk and mix until incorporated. Scrape the sides of the bowl.

Turn the mixer back up to medium-high speed and beat the buttercream for an additional 6-7 minutes.

If the buttercream is too thick, add in a bit of heavy cream or milk, one teaspoon at a time until you reach the desired consistency.