Melt the chocolate using your preferred method (either in a double boiler or in a microwave). Set aside to cool.
While the chocolate is cooling, in a clean, medium-sized bowl (or to the bowl of a stand mixer) add the whipping cream. Whip the cream using a whisk, a stand mixer, or a hand mixer until you achieve stiff peaks (this will take about 4 minutes with a hand-mixer). Be careful not to over-whip. Place the bowl of whipped cream in the fridge to be used later.
In a double boiler on the stove, add your eggs and sugar. Because we are not cooking the filling, we want to bring the eggs to a temperature of 160°F to rid them of any bad bacteria that they might be carrying. To do this, continuously whisk the egg mixture in the double boiler until a thermometer reads 160°F. Be careful. You want to make sure you keep whisking the entire time, or else the eggs might begin to cook and coagulate (YUCK!).
When the egg mix has come to temperature, take it off the heat and set aside to cool for about 5-10 minutes.
After 5-10 minutes, pour the cool but still melted chocolate into the egg mixture and whisk well until totally combined. Set aside.
In a separate, clean, large bowl, add your butter and salt. Using a hand mixer, beat your butter and salt together until the butter is light and fluffy and you can verify that it is about room temperature meaning there are no clumps of hard, cold, butter. Once you ensure that the butter is completely creamy and the salt is incorporated, slowly stream in the chocolate/egg mixture from earlier, mixing the entire time. Once all of the chocolate/egg mix is added in, beat for an additional minute or so to help keep the mixture light and fluffy.
Take the whipped cream out of the fridge and gently fold in half of it into the mixture. Once the first half is totally incorporated, add the second half and fold in GENTLY, until no white streaks of cream remain.
Dollop out spoonfuls of the filling into the cooled graham cracker crusts. Fill the crust entirely but keep the filling level, do not pile it higher than the edges of the tart.
Place tarts in the fridge for at least an hour.
Move on to making the marshmallow fluff.