8 tablespoons (4 ounces) unsalted or salted butter, slightly softened but still cold, cut into chunks
Thoroughly mix the cream, sugar and salt, if using, in a small saucepan. Wipe the sides of the pan clean with a silicone spatula and rinse the spatula (any sugar crystals clinging to it might crystallize the frosting when you use it later). Cover the pan, set over medium heat, and heat until the mixture is bubbling all over. Uncover the pan, adjust the heat so that the mixture boils actively but not too furiously, and cook, without stirring, for 1 minute. Use the spatula to scrape the hot mixture into a medium stainless steel bowl. Let cool, without stirring, until barely lukewarm, about 45 minutes.
Set the bowl in a larger bowl of cold water and ice cubes and add the vanilla to the frosting, then, with the electric mixer, gradually beat in the chunks of butter and continue to beat until the frosting is smooth and fluffy. If it is not thick or stiff enough, keep it in the ice water and continue to beat until it thickens, or chill the bowl for 15 minutes in the fridge and beat it again. Or, if the frosting is too stiff or cold, set the bowl in a larger bowl of hot water until the frosting starts to melt around the bottom, then beat it or stir briskly with a spatula until it reaches the desired consistency.