Starbucks' $1.25 cold foam is the chain's most popular drinks add-on, and one of its most expensive. This is how it's made.

• Cold foam is now Starbucks' most popular beverage modification.
• It's made by frothing nonfat cold milk and is added to iced drinks like cold brews.
• The $1.25 add-on was first rolled out at regular Starbucks stores in 2018.

Customers can't get enough of Starbucks' cold foam. It's now the coffee chain's most popular add-on, and it's pushing up the Siren's revenues.

The "velvety" foam is made by frothing nonfat cold milk "until it is smooth, creating layers of creamy texture and flavor without the cream," Starbucks says.