Combine dark muscovado, water, ground cinnamon into a cup.
Microwave for 20 to 30 seconds (or heat up on stovetop with low heat) until syrup is slightly thickened and all ingredients combined. Set aside and cooled.
Brew espresso shots.
In a shaker, pour in brown sugar syrup, ice, then the espresso shots. Close the lid tightly. Shaken with up and down motion 10 times.
Assemble the drink in a tall glass by adding the shaken espresso, add more ice if desired, and topped with oat milk.