Start by brewing two fresh shots of espresso using your espresso machine, Moka pot, or pod-based brewer. It is important to use the coffee while it is still hot so that it dissolves the brown sugar effectively.
In a cocktail shaker or a mason jar with a tight-fitting lid, combine the hot espresso, brown sugar, and ground cinnamon. Add one cup of ice. Seal the lid tightly and shake vigorously for about 15 to 20 seconds. You want to see a thick layer of foam forming on the top.
Fill a tall glass with fresh ice cubes. Do not reuse the ice from the shaker, as it will have melted slightly and become rounded, which can lead to a more diluted drink.
Strain the shaken espresso mixture over the fresh ice in your glass. Make sure to pour quickly so all that delicious, airy foam makes its way from the shaker into the glass.
Gently pour your oat milk or preferred milk over the top of the espresso. Watch the beautiful marble effect as the milk swirls into the dark coffee. Give it a gentle stir if you like, and enjoy immediately!