In a large bowl, mix the flour, unsweetened cocoa, and sugar together in mixing bowl. With a large fork, cut in the butter until crumbly and pea-sized and smaller pieces. Pour in the cold water all at once, mix just until dough ball forms, and roll out into a pie plate. Set aside for filling.

Preheat the oven to 350 F. In a saucepan over medium-high heat, melt the chocolate chips and butter until barely melted. Remove from heat, stir into sugar, flour & cocoa powder until well-combined. Whisk in the eggs thoroughly and stir in vanilla. Pour filling into the prepared crust and bake for 35-40 minutes. The middle may still be soft but will firm up after cooling for a few minutes (about 15 to 20 minutes). Stir the marshmallow fluff in the jar to help with spooning it out. Spoon marshmallow fluff over the warm pie. For the second layer, in a saucepan over medium-high heat, stir together powdered sugar, cocoa, milk, butter, and vanilla until it starts to steam/simmer. Remove from heat and spoon over the marshmallow fluff layer. Finish with chocolate shavings or sprinkles. Let the pie cool and serve slightly warm or at room temperature.