Place butter in the bowl of a stand mixer (or use a large bowl and an electric mixer) and beat on low-speed until creamy and smooth. Scrape the sides and bottom of the bowl.

Add 2 cups (250g) of powdered sugar, cocoa powder, vanilla extract, ⅛ teaspoon salt, and 3 Tablespoons of heavy cream and turn mixer to low speed. Stir until well-combined.

Gradually (with mixer on low-speed) add remaining powdered sugar. The icing may appear pretty dry, this is OK.

Add remaining heavy cream or milk, one tablespoon at a time. Start with mixer on low-speed and gradually increase speed to medium-high, beating on this speed for several seconds to whip each Tablespoon of cream into the batter. Add additional tablespoons of cream until a smooth, pipe-able consistency is reached.

I recommend piping icing over your cupcakes or frosting your cake immediately. See notes in the post on how to store buttercream and use at a later date.