Place the chocolate and butter in a heatproof bowl.

In a saucepan over medium heat, bring the heavy cream and salt to a low simmer (160-170°F). Pour over chocolate and butter. Let stand for 3 to 5 minutes, then whisk until smooth and chocolate is completely melted.

Cover with plastic wrap and let cool to room temperature, about 45 minutes.

Place in the bowl of a stand mixer fitted with the whisk attachment and whip on medium-high speed until it's light in color and texture.

Use as you would frosting.