Adjust your rack so it's on the bottom third of the oven and preheat the oven to 375 degrees. Butter a 9-inch springform pan. Wrap the bottom of the pan with aluminum foil to seal it from any water getting inside the pan.
Set up a double boiler (heat water in a pot then place a larger metal/heat resistant bowl on top that does not touch the water). Add the chocolate, fudge packet and butter to the metal bowl and place on top of the pot of hot water. Stir until melted and combined then set aside.
In a small mixing bowl, beat the egg yolks with 3/4 cup of the sugar (setting aside the remaining 1/4 cup sugar for later) on high speed with your electric mixer for a few minutes until the mixture forms a wide band when you lift the beaters up and the sugar is dissolved. Add in the chocolate mixture and beat on low until combined and set aside.
Before beating the egg whites, thoroughly clean your beaters and use a clean bowl (this is important--any leftover residue from the previous step will turn the egg whites into a liquid).
On a moderate speed, beat the egg whites and gradually add the remaining 1/4 cup of sugar.
Tip: if you have time, put the bowl and beaters in the freezer for a few minutes to get them cold--this will help make your egg whites fluffy in a short amount of time! When the egg whites and sugar form a fluffy texture that holds itself, fold the egg whites to the chocolate mixture about 1/3 at a time.
Pour the mixture into the springform pan and place the pan in the larger pan. Place both in the oven and fill the larger pan with water.
Bake at 375 degrees for 15 minutes, then lower the temperature to 350 degrees and bake for another 15 minutes, and finally another 30 minutes at 325 degrees (a total of one hour).
Remove both pans from the oven and carefully take out the springform pan from the larger pan with the water. Carefully remove the aluminum foil and the other ring of the springform. Let the cake cool for about 30 minutes.
Using the same double-boiler technique, add the heavy cream and chocolate until its both melted and combined. Pour evenly on top of the cooled cake.
Using a large knife, chop the pistachio nuts and drizzle them on top of the warm ganache. The sprinkle about 1/2 teaspoon of the Himalayan pink salt. Let the ganache cool before cutting and serving!
Before you start, make sure you have a 9-inch springform pan (like this one here) and a larger, deep bake-safe (metal, ceramic, glass, etc) pan that the 9-inch springform can fit into. We'll be putting the smaller pan in the larger one and filling the larger pan with water while the cake cooks! You will also need a metal bowl and an electric mixer...hand-held is fine!