How do you make marshmallow cold foam at home?

Whisk 1 tablespoon of marshmallow fluff into ¼ cup of cold half and half until it thickens and holds soft peaks. A small handheld milk frother works great. So does a Mason jar with a tight lid and 30 seconds of shaking. The foam will sit on top of the drink and slowly melt into the cold brew as you sip.

Why cinnamon if there's no real fire?

Cinnamon is what makes this taste like a campfire instead of just an iced mocha with marshmallows. It brings the warm, aromatic edge that pulls the whole drink toward the bakery-side of s'mores (think graham cracker), even though there's no smoke involved. A pinch is enough. More than that turns it into a horchata.

Can I skip the half and half in the cold foam?

You can, but the foam won't hold as well. Half and half whips thicker than milk thanks to its fat content. If you're dairy-free, full-fat oat milk is the closest substitute, though it won't get as stiff. Skim or 2 percent will flatten almost immediately.