Arrange two shallow dishes on a flat surface. To the first, add chocolate syrup in a circular pattern approximately the size of the rim of the glass. To the second, pour the crushed cracker crumbs.
Swirl the rim of a chilled martini glass in the chocolate syrup, then immediately dredge in graham cracker crumbs. Set aside.
Fill a martini shaker with ice. Add chocolate liqueur, crème de cacao, and vodka. Shake vigorously until chilled, about 20 seconds. Strain into the prepared glass and garnish with a toasted marshmallow.