In the U.S., heavy cream must have at least 36% milk fat while whole milk has between 0.5% and 3.5% milk fat, so one easy way to readjust milk’s fat content is to add butter. Depending on the brand, butter typically has 80% to 84% milk fat, so simply mixing ¾ cup of milk and ¼ cup of melted butter will create the perfect heavy cream substitute.