As @RickBeer said you need to look at the recipe and see how the flavors are getting into the beer. You could use the St. Patrick's Stout but you would need to be able to get the graham cracker flavor into it and a mash is the only way I see doing that. Also, true chocolate flavor is not easily achieved, I have done it in a Mead with Dutched Cocoa Powder but not in a beer. The bakers chocolate may work but you would need a boil to use it and I would be afraid of the fat from the cocoa butter. If I were doing this recipe I would try Cocoa Nibs in secondary to achieve the chocolate flavor rather than the bakers chocolate. You could add the marshmallow flavor using the vodka without an issue. Also, if you look at the recipe there is a good amount of Lactose in the grain bill. Lactose is not fermentable by yeast so it leaves a sweetness that you would also want in this beer.
You could try this but there would be no guarantees it would work.
Mash 8oz of 2 Row malt with 8 oz of crushed Graham Crackers at 150 degrees for 30 minutes in 2 qts of water.
Strain off resulting wort and add water to achieve 1.5 qt of wort.
Add 6 oz of lactose and boil for 15 minutes or until you have 1 qt of wort remaining.
Remove from heat and add your St. Patricks Stout HME and ferment as usual.
After 2 weeks transfer to another LBK and add 4oz of crushed Cocoa nibs and 2 oz of marshmallow vodka.
Note: soak 1 vanilla bean in the 2 oz of marshmallow vodka at beginning of fermentation
Let sit in secondary for at least 1 week, 10 days may be better.
Bottle and condition as usual