To make the simple syrup, add water and brown sugar to a small saucepan and mix.
Turn to high heat and add cinnamon stick to the saucepan.
Bring the mixture to a boil and then turn the heat to low.
Let the simple syrup simmer for 5-10 minutes.
Remove from heat and let cool. Set aside.
To make the drink, place 5-6 ice cubes in a cocktail shaker or mason jar with a cover.
Add vodka, espresso, Kahua, and 1 tablespoon of simple syrup. Shake vigorously for at least 30 seconds.
Pour espresso martini into a martini glass while holding the ice cubes from falling into the glass.
Place the oat milk into a mason jar or tall glass. Using a frother, froth the oat milk until light and airy.
Garnish each glass with oat milk and an espresso bean or powder.