This recipe for raspberry buttercream takes the cake as a filling in our fondant-covered XOXO white wedding cake.
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 and 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. Raise speed to medium-high; whisk until stiff, glossy peaks form and mixture is 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 raspberry puree.
Switch to paddle; beat on low to reduce air bubbles, 3 to 5 minutes.