Poke a marshmallow onto a skewer, making sure the skewer pierces at least halfway through the marshmallow. Toast the marshmallow evenly on all sides, including the bottom, over medium heat. (Alternately, preheat the broiler and evenly space the marshmallows on a baking sheet. Broil until slightly charred and puffed.)
Place the marshmallow toasted-side up on a plate and let cool slightly, about 30 seconds. Carefully remove the inside of the marshmallow with a small metal spoon, being careful not to rip the marshmallow; transfer the insides to a microwave-safe bowl. Repeat with the remaining marshmallows. Place the hollowed marshmallow shells in the fridge until slightly firm, about 5 minutes.
Microwave the bowl of marshmallow insides until slightly melted, about 30 seconds. Mix in the prepared chocolate pudding and liqueur, if using, until well combined.
Transfer the mixture to a resealable plastic bag or pastry bag and trim the tip off. Pipe the mixture into each marshmallow. Chill until the mixture becomes slightly firm, about 10 minutes.
When ready to serve, drizzle with chocolate syrup, then sprinkle with crushed graham crackers and top with whipped cream.