In the heatproof bowl of an electric mixer set over a saucepan of simmering water, combine egg whites, sugar, and salt.
Cook, whisking constantly until sugar has dissolved and mixture is warm to the touch (about 160 F).
Attach the bowl to a mixer fitted with its whisk attachment. Beat the egg white mixture on medium speed until fluffy, about 10 minutes.
Continue beating on high speed until the mixture is fluffy and cooled, about six minutes.
Switch to the paddle attachment. With the mixer on medium-low speed, add Biscoff spread a little at a time.
While still on medium-low speed, add the butter several tablespoons at a time, beating well after each addition. (If the frosting appears to separate after the butter has been added, beat on medium-high until smooth again, 3-5 minutes more.)
Pipe onto cooled cupcakes.