To make Chocolate Sauce: Combine water and sugar in a saucepan over medium heat. Bring to a boil, whisking until sugar is dissolved. Whisk in cocoa, pumpkin pie spice, cayenne pepper and salt. Simmer, whisking ocassionally, until slightly thickened about 3 minutes.
Remove from heat and add vanilla, then cool (syrup will continue to thicken as it cools). Syrup can be made ahead and refrigerated until ready to use.
Toasted Marshmallows: Preheat broiler to high. Place marshmallows on a sprayed sheet tray, broil for about 1 minute until golden brown. Remove from the oven and set aside.
Adult S'more Milk Shake: Combine toasted marshmallows, chocolate and vanilla ice creams, chocolate liqueur, vanilla vodka and vanilla extract in a blender. Blend until smooth and creamy.
Rim each glass in the chocolate sauce, followed by graham cracker dust. Pour milkshake into prepared glasses. Garnish with a toasted marshmallow.