Combine brown sugar, water, and vanilla in a small saucepan. Bring to a simmer over medium-high heat and stir continuously for 2-3 minutes. Remove from heat and set aside.
Add syrup, ice, cinnamon, and espresso in a shaker and shake vigorously for about 1 minute until frothy.
Pour the entire shaker into a glass, add more ice if needed, and top with oatmilk!
*Depending on how sweet you like your coffee, you can reduce the amount of brown sugar to as low as a 1/2 tablespoon and the flavor will still come through. Just also reduce the water and vanilla accordingly.