This frosting has an appealing mousse-like texture. Try it between layers of any angel food cake, a moist sponge cake, or Devil’s Food Cake Cockaigne.

Whisk together in a medium heatproof bowl, preferably stainless steel:

Set the bowl in a large skillet of barely simmering water and heat, stirring constantly, until the mixture registers 160°F on an instant-read thermometer. Remove from the heat and stir in:

Stir until the chocolate and butter are melted and the mixture is smooth. Set the bowl in a larger bowl of ice water and beat on high speed until the frosting holds a shape, 7 to 10 minutes. This keeps, refrigerated, for up to 4 days.