The syrup keeps in an airtight glass jar in the fridge for up to 2 weeks, or frozen in an ice cube tray for 3 months. Reheat individual syrup portions in the microwave for 15–20 seconds before assembling. Never reheat the finished drink — the oatmilk foam will collapse. For a spicier version, add a pinch of nutmeg and cloves to the syrup while it simmers. No espresso machine? Use 1/2 cup strong cold brew concentrate and skip the espresso pour.