Line an 8-inch square baking pan or 9-inch square baking pan with parchment paper, leaving an overhang on the sides to lift the cake out (makes cutting and serving possible!). Set aside.

Place chopped chocolate in a bowl. Heat the cream in a small saucepan until it begins to gently simmer. (Do not let it come to a rapid boil—that’s too hot!) Pour over chocolate and let it sit for 2–3 minutes to gently soften the chocolate. Slowly stir until completely combined and chocolate has melted. Cover and refrigerate as you make the meringue. Ganache can be prepared, covered, and refrigerated for up to 3 days before using.

Place egg whites, sugar, and cream of tartar in a heatproof bowl. Set bowl over a saucepan filled with 2 inches of simmering water. Do not let it touch the water. (You can use a double boiler if you have one.) Whisk constantly until sugar is dissolved and mixture has thinned out, about 4 minutes. If you’d like to be precise and use a candy or instant-read thermometer, the temperature should reach 160°F (71°C). Add the vanilla extract, then using a handheld or stand mixer fitted with a whisk attachment, beat on high speed for 5–6 minutes until stiff glossy peaks form. Remove 1 heaping cup (just eyeball it), cover tightly, and refrigerate for up to 2 days. This will be the topping.

If the marshmallow meringue and ganache are still slightly warm as you assemble the cake, that’s ok. Spread a super thin layer of marshmallow meringue on the bottom of the pan. Doesn’t need to be even or perfect; it’s just so the graham cracker base has something to stick to. Do the best you can with the marshmallow meringue because it’s sticky. Spread it slowly. Top with a layer of graham crackers. You’ll need to break them apart to evenly arrange them; see my video below for a visual. Top with a layer of marshmallow meringue, then another layer of graham crackers, then a layer of chocolate ganache, then a layer of graham crackers, then another layer of marshmallow meringue. Repeat for a total of 6 layers of graham crackers, 4 layers of marshmallow meringue (including the thin bottom layer), and 3 layers of chocolate ganache. The top layer will be chocolate for now. Layers listed out: super thin marshmallow meringue, graham crackers, marshmallow meringue, graham crackers, chocolate, graham crackers, marshmallow meringue, graham crackers, chocolate, graham crackers, marshmallow meringue, graham crackers, chocolate.

Cover tightly and refrigerate for at least 12 hours and up to 2 days.

Remove reserved marshmallow meringue from the refrigerator. If it seems deflated, whip it a few times with a fork or whisk.

Remove cake from the refrigerator, then remove from the pan using the overhang on the sides. Peel off the parchment paper (or don’t—if you trust yourself torching the top without the paper lighting up!) and place onto a heatproof serving platter or cutting board. Spread marshmallow meringue on top. Use a spoon to create peaks and swirls. Serve immediately OR torch it with a kitchen torch and serve immediately OR loosely cover and refrigerate for up to 1 hour before torching/serving.

Cover and store leftovers in the refrigerator or freezer for up to 2 days. Topping may begin to wilt.