How to Make It
Combine milk, cocoa, powdered milk, and instant coffee in a medium-sized saucepan. Over medium heat, bring the mixture to a simmer, stirring occasionally. Reduce the temperature to low to keep the mixture warm.
In a heatproof bowl, whisk together the sugar, cornstarch, salt. Add the egg yolks and beat until the mixture is smooth and pale yellow.
Measure out 1 cup of the warm milk mixture and slowly stream it into the egg mixture while whisking.
Bring the heat under the saucepan back to medium, and while whisking constantly, slowly stream in the egg mixture. Continue whisking constantly over medium heat until the mixture thickens to the consistency of a thick pudding, 5 to 6 minutes.
Remove from heat and whisk in butter and vanilla extract. Allow to cool for 5 minutes, then pour the mixture into the piecrust. Cover and chill for 12 hours.
Dollop the surface with marshmallow creme and smooth across the pie surface. Garnish with crushed peppermint candies, if desired.