In the heatproof bowl of an electric mixer set over (but not touching) simmering water, combine egg whites and sugar. Cook, whisking constantly, until sugar has dissolved and mixture is warm to the touch (about 160 degrees).
Switch to the paddle attachment. With mixer on medium-low speed, add butter, several tablespoons at a time, beating well after each addition. (If frosting appears to separate after all the butter has been added, beat on medium-high speed until smooth again, 3 to 5 minutes more.) Beat in vanilla. Beat on lowest speed to eliminate any air bubbles, about 2 minutes. Using a rubber spatula, gently fold in passion fruit curd until smooth.