Make this delicious coffee buttercream frosting to go with our chocolate-and-coffee Opera Cake. Serve it to guests at the end of the night after the wedding cake, and they'll be sure to thank you.
Place espresso powder in a small cup; add the boiling water, and stir to dissolve. Let cool.
Bring a saucepan with about 2 inches of water to a simmer. Combine sugar and whites in a large heatproof mixer bowl set over (not in) simmering water. Whisk until whites are warm to the touch and sugar is dissolved, 2 to 3 minutes.
Place bowl on mixer stand; whisk on low speed until mixture is foamy. Whisk on medium-high until stiff, glossy peaks form and mixture is completely cooled completely, about 10 minutes.
Reduce speed to medium-low; add butter 2 tablespoons at a time, whisking to incorporate fully well after each addition (if buttercream appears curdled, at this point simply beat until smooth). Whisk in vanilla and espresso mixture.
Switch to paddle attachment; beat on lowest speed to reduce air bubbles, 3 to 5 minutes.