In a saucepan with the heat turned off, mix together the cold milk and cornstarch until it is well combined and there are no lumps.
Add in the creamer, coffee, salt, cocoa powder, sugar, whole egg, and yolks and whisk until well combined. (You can use an immersion blender to speed up the process).
Turn on the heat and add in the heavy cream and chocolate. Stir constantly until the mixture thickens visibly.
Strain the mixture through a fine mesh strainer into a bowl and add in the butter. Stir until the butter melts. Place plastic wrap on top, directly touching the pudding, and chill completely.
While the pudding chills, crush the graham crackers and stir in the melted butter and salt.
Take small containers and layer the graham crackers and chocolate pudding. Top with the toasted marshmallows.