Begin by placing your martini glasses into the freezer until you are ready to use.
Prepare the brown sugar simple syrup by combining the brown sugar and water in a small saucepan.Set to medium heat. Let the mixture to come to a simmer, allowing the brown sugar to dissolve.
Once dissolved, simmer for an additional minute, then set aside until ready to use.
To your pitcher, add the vodka, kahlúa, espresso, and 1 ounce of the brown sugar simple syrup.
Gently mix, cover with a lid, and refrigerate until ready to use.
To prepare the martinis, add a handful of ice to your cocktail shaker, followed by half of the espresso martini mixture.
Shake vigorously for at least 30-45 seconds.
Using a small strainer, pour the espresso martini mixture into two of the frosted glasses.
Repeat these steps to shake up the last two martinis.
Garnish with an espresso bean and enjoy right away.