Melt chocolate in a heatproof bowl set over a pan of simmering water, stirring occasionally. Let cool to room temperature, 25 to 30 minutes.
Meanwhile, put butter, confectioners' sugar, and salt into 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. Add melted chocolate, and mix until combined, scraping down sides of bowl as needed. Use immediately, or refrigerate in an airtight container up to 3 days; bring to room temperature, and stir before using.
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