Add the cold brew, brown sugar syrup, and ice to a glass. Give it a quick stir to combine, then set aside.
In a separate small cup, combine the eggnog and a pinch of salt. Froth using a handheld milk frother until thick and foamy (about 10–15 seconds).
Pour the salted eggnog cold foam over the cold brew.
Top with a sprinkle of nutmeg and enjoy!