CHOCOLATE: Pour the chocolate chips in a large heat safe bowl. Scald 1 cup of the heavy cream. Pour the hot heavy cream over the chocolate chips and cover the bowl with a clean lid to a pan (so no heat can escape). Let stand for 5 minutes and then stir until smooth. If needed, heat in the microwave in bursts of 10 seconds, stirring in between for 10 seconds until chocolate mixture is completely smooth. Place in the fridge until ready to use.
MARSHMALLOW: Pour the remaining 2 cups heavy cream, powdered sugar, and vanilla in the base of a stand mixer fitted with a whisk attachment. Beat on low for 30 seconds to combine and then gradually increase the speed until at full speed. Beat at full speed, watching carefully until soft peaks form (about 1 minute). Soft peaks = when you turn your whisk upside down, the peaks are just starting to hold but falling to a side slightly. Add in all of the marshmallow fluff and slowly beat to combine. Increase the speed to medium high until ingredients are combined and peaks are firm (about 30 seconds). Set aside.
ASSEMBLE: Line a 9 x 9 inch baking pan with parchment paper (don't skip or bars won't come out easily). Spread just a tiny bit of the marshmallow cream on the bottom (just enough to hold graham crackers in place). Spread an even layer of graham crackers. Now spread 1/4th of the marshmallow cream on top. Smooth with the back of a spoon until it is in one even layer. Now add 1/3 of the chocolate layer on top. Smooth with a butterknife until smooth and in one even layer. This takes some patience! Now another layer of graham crackers. Next another 1/4th of the marshmallow mixture. Next another 1/3 of the chocolate mixture. Now one more layer of graham crackers, one more 1/4th of marshmallow mixture, remaining 1/3 of chocolate. Add one last layer of graham crackers and the last 1/4th of the marshmallow mixture and you should be at the top of the pan. Cover the pan tightly and place in the fridge for at least 8 hours and preferably 12 hours.
MERINGUE: Place the egg whites, sugar, and cream of tartar in a heatproof bowl. Set the bowl over a saucepan filled with two inches of simmering water over medium heat. Don't let the bowl touch that water. Whisk constantly with a whisk until sugar is dissolved completely and the mixture has become thinner. Now transfer to a stand mixer (or use hand mixers) and beat until stiff peaks form, about 5 minutes at high speed. Using a spatula spread evenly over the top of the cake. Use a fork to run over the top quickly creating small rough ridges and peaks. IF you have a kitchen torch, torch the peaks until lightly browned. Cut into squares and serve immediately.