In the bowl of an electric mixer fitted with the whisk attachment, whisk together egg whites and sugar until combined.Advertisement
Set bowl over (but not touching) simmering water; continue whisking until sugar is completely dissolved and mixture has reached 140 degrees on a instant-read thermometer, 6 to 8 minutes.
Return bowl to electric mixer and beat on high speed until thick, about 3 minutes. Add cream of tartar and continue beating until stiff peaks form, about 3 minutes more. Add amaretto liqueur and beat until just combined.