CHOCOLATE: Pour the chocolate chips in a large heat-safe bowl. Scald 1 cup (240g) of the heavy cream (I add it to a glass measuring cup and microwave for 45-90 seconds). 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. This may take a few minutes. If needed, heat the mixture in the microwave in bursts of 10 seconds, stirring in between for 10 seconds until the chocolate mixture is completely smooth. Set aside at room temperature until ready to use. (Ganache needs to be completely cooled to room temperature before adding to the cake or it will affect the texture of the marshmallow cream!)
MARSHMALLOW CREAM: Pour the remaining 2 cups (480g) 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 2-4 minutes). 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 up to 1 minute). Set aside.
ASSEMBLE: Line a 9x9-inch baking pan with parchment paper. (Don't skip this step, or bars won't come out as nicely.) 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 room temperature chocolate ganache on top. Smooth with a table knife until smooth and in one even layer. (This takes some patience!) Now another layer of graham crackers, and then another 1/4th of the marshmallow mixture. Next, another 1/3 of the chocolate mixture and 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.
OPTIONAL 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 bright white. (You can tell the sugar has dissolved by pinching off some of the mixture and rubbing it between your fingers, if you can feel a lot of sugar granules, keep mixing!) Now transfer to a stand mixer (or use a hand mixer) and beat until stiff peaks form, about 5 minutes at high speed. Using a spatula add dollops of the meringue all over the top of the cake. Gently coax the meringue to top the cake with the back of a spoon -- This takes some patience to not lift up the top layer! Use a fork to run over the top, creating small rough ridges and peaks. If you have a kitchen torch, singe the peaks until lightly browned (move the kitchen torch along the meringue quickly-- it will toast it very fast). Use the overhang of parchment to carefully remove bars from pan onto a cutting board. Use a sharp chef’s knife, run the knife under warm water, dry on a kitchen towel, and make decisive cuts. Repeat this process for each cut made.
STORAGE: This cake is best eaten on days 2,3, or 4 and after that it does begin to become a bit watery. Keep the cake in the fridge and serve it directly from the fridge. Return any leftovers promptly to the fridge. This cake isn't a great candidate for freezing and thawing -- there is so much dairy (heavy whipping cream) in it that it can become grainy and watery.
Note 1: 9x9-inch pan: While it may not seem like a big deal to use a slightly smaller pan, this cake doesn't work well or fit nicely in an 8x8-inch or smaller pan. The 9x9-inch pan is filled to the very brim and allows for the graham crackers to be placed in a nice even layer with minimal breaking.Note 2: Chocolate chips: 12 ounces is about 2 full cups (345g). Use high-quality chocolate chips for the best results. You’ll get both a better melt and a better flavor. My favorite is Ghirardelli® or Guittard®. Cheaper brands can have a waxier content that is far less flavorful and won’t melt into a smooth ganache as nicely.Note 3: Marshmallow creme: Jet-Puffed Marshmallow Creme® (not sponsored) is our favorite to use. To easily remove the marshmallow creme (which is very sticky!) from the jar, run a spoon under hot water for about 20 seconds. Dry it off and scoop out all of the creme from the container and into the bowl of the stand mixer.