In a small bowl, combine the brown sugar, hot water, and ground cinnamon, stirring until the sugar is completely dissolved to create a syrup.
Brew your espresso shots and pour them into a cocktail shaker or a jar with a tight-fitting lid.
Add the brown sugar cinnamon syrup and ice cubes to the shaker with the espresso.
Shake vigorously for about 15-20 seconds until the mixture is well-chilled and frothy.
Fill a tall glass with fresh ice, pour in the oat milk, then strain the shaken espresso mixture over the top.
Stir gently before drinking to combine the layers and enjoy immediately.