Melt the butter and mix it with the graham cracker crumbs and the sugar.
Combine the whipping cream, butter, and chopped chocolate in a microwave safe bowl.
Microwave for 2 minutes and then stir to form a chocolate cream.
In a second bowl, whisk together the sugar, cocoa powder, flour, and salt.
Add the chocolate cream to the dry ingredients, folding it in until combined.
Whisk in the egg and egg yolk.
Preheat the oven to 350F and place mason jar lids on a baking sheet covered in parchment paper.
Press 3 teaspoons of graham cracker mixture into each lid to for the crust. It will feel loose and crumbly until fully baked. Bake for 10 minutes.
Add 1 1/2 tablespoons of the chocolate mixture to each tart. Bake for 10 minutes.
Cool tarts in the refrigerator for 10 minutes.
Pop the tarts out of their lid "molds" and place them back on the baking sheet with parchment paper.
Arrange marshmallows on top, and broil in the top rack of the oven until they reach desired level of browning.
*For less dense chocolate filling use 1 1/2 egg yolks instead of two.