Combine brown sugar, water, cinnamon, and sea salt in a saucepan. Bring it to a boil and simmer for 10 minutes. Once cooled, store in a sealed jar in the fridge for up to 4 weeks.
Prepare 2 shots of espresso or a strong coffee equivalent. Freshly brewed espresso makes the coffee richer and more aromatic.
Pour the espresso and a portion of syrup over ice in a shaker or mason jar. Shake vigorously until frothy and well-chilled.
Fill a glass with fresh ice, strain the shaken coffee mixture over it, and top with oat milk. Stir gently and enjoy your creamy, frothy iced coffee.