Preheat oven to 350°F. Grease your tart pan and set aside.
Make the crust: pulse graham crackers in a food processor until fine crumbs form, or crush in a zip-lock bag. Mix with sugar and melted butter until it resembles wet sand.
Press crust firmly and evenly into the bottom and up the sides of your greased tart pan.
Bake for 10 minutes until lightly golden. Remove and let cool completely. This is the LAST time your oven is involved!
Make the ganache: add heavy cream and butter to a saucepan over medium heat. Bring to a gentle simmer. You want it just barely bubbling. Do NOT let it come to a full boil.
Place chopped dark and milk chocolate in a heatproof bowl. Pour the hot cream mixture over the chocolate and let it sit undisturbed for 3 full minutes.
Stir slowly from the center outward until completely smooth and glossy.
Pour ganache over the cooled crust and spread evenly. Refrigerate for at least 2 hours. The longer the better. Overnight is AMAZING.
Make the marshmallow cream: set up a double boiler (heatproof bowl over a pot of simmering water). Add egg whites, sugar, and cream of tartar. Whisk constantly for 4-5 minutes until sugar is fully dissolved and mixture is warm to the touch.
Transfer to a stand mixer and beat on high for 10 minutes until glossy, stiff peaks form.
Spoon or pipe the marshmallow cream over the set tart.
Toast with a kitchen torch until golden and caramelized. DO NOT put this in the oven. The ganache will melt!
Slice, serve, and watch everyone lose their minds.