In a small frying pan or pot, combine the syrup ingredients over medium-high heat, whisking to mix. Once it begins to simmer, reduce the heat to medium-low and simmer for about 3-5 minutes, stirring often, until the sugar dissolves completely and the syrup thickens slightly. Remove from heat and set aside.
Brew two shots of espresso or about 1/2 cup of very strong coffee. Add the espresso to a mason jar along with 3 tbsp of syrup and a few ice cubes, then shake vigorously for 1-2 minutes.
Add additional ice to each glass, pour the espresso mixture into the glasses, and top with oat milk. Stir well and enjoy!