Put chocolate in a heatproof bowl set over a pan of simmering water. Turn off heat. Let stand, stirring occasionally, until chocolate has melted, about 15 minutes. Let cool to room temperature, 25 to 30 minutes.Advertisement
Put butter, sugar, and salt in the bowl of an electric mixer fitted with the paddle attachment; mix on medium-high speed until pale and fluffy, 3 to 4 minutes. Reduce speed to low. Mix in melted chocolate until combined, scraping down sides of bowl as needed, 1 to 2 minutes. Chill, stirring often, just until frosting is firm enough to spread or pipe, 10 to 15 minutes. Use immediately.