Step 1
In a large saucepan, combine the sugar, cornstarch, and salt. Slowly whisk in the milk, half-and-half, and chocolate. Cook over medium high heat until thickened and bubbly, roughly 5–8 minutes, stirring constantly. Reduce the heat to low and cook 2 minutes longer. Remove from heat.
Step 2
Place the yolks in a small bowl; whisk a small amount of the hot filling into the yolks. Pour the mixture into the pan, stirring constantly. Bring to a simmer; cook and whisk 2–3 minutes longer. Remove from the heat.
Step 3
Carefully stir in the butter and vanilla, then spoon the mixture into the pastry shell. Allow to cool to room temperature, then refrigerate for at least 3 hours or overnight. Top with marshmallows. Use a kitchen torch to toast, if desired. Sprinkle graham crumbs over the top for garnish.
Step 4
Excerpted with permission from <a href="http://www.amazon.com/gp/product/1440565279/ref=as_li_qf_sp_asin_il_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=1440565279&linkCode=as2&tag=craftfocom-20">"The S'mores Cookbook: From S'mores Stuffed French Toast to a S'mores Cheesecake Recipe, Treat Yourself to S'more of Everything,"</a> by Susan Whetzel and published by Adams Media.
