To a large microwave-safe bowl, add the chopped chocolate. Heat in 30-second increments, stirring in between until smooth and creamy. Set aside to cool slightly (3-5 minutes).
In a separate small bowl combine hot water, cocoa powder, vanilla, and salt. Mix well.
Add the hot water mixture to the melted and cooled chocolate. Mix well. (It should have a ganache-like texture.)
To a large mixing bowl, add heavy cream. Start mixing with an electric mixer at the lowest speed, and then gradually increase speed to medium until the cream starts to get thicker. (The gradual progression prevents you from spraying yourself with cream.)
When the cream starts to thicken, stop the mixer. Slowly add in the confectioners' sugar and then resume mixing again, starting with low speed and increasing to medium speed and eventually moving to a high speed.
The cream will thicken and start to peak. Mix on high until the cream is thick and stiff. This happens quickly, the stage between soft and stiff peaks, so watch it closely.
Add half of the whipped cream mixture into the chocolate mixture and gently fold until completely combined. (You should not see any white streaks.) Chill the other half of the whipped cream mixture for topping later. (We put ours in a piping bag.)
Spread the chocolate mixture on top of the graham cracker crust.
Refrigerate the pie for at least 2 hours, or until set. Serve topped with the remaining whipped topping. I piped mine with an 8B tip from Wilton. You can also spread a smooth layer or dollop the whipped topping.