To make the graham crust, stir together the melted butter and graham crumbs in a large bowl. Once they are all moistened, stir in the coconut oil. I find that a little coconut oil helps keep the tart crust together. Press the crust into a tart pan that has a removable bottom (if desired so you can display it). Stick it in the fridge while you make the pumpkin layer.
Add the mascarpone and cream cheese to the bowl of your electric mixer and beat on low speed for 30 seconds. Add in the pumpkin and the condensed milk, mixing until you have a creamy batter-looking mixture. With the mixer on low speed, mix in the sugar and vanilla until combined. Pour the pumpkin into the graham crust and spread it around evenly. Refrigerate for 30 minutes.
To make the ganache layer, add the chopped chocolate to a bowl. Heat the heavy cream over low heat until it warms and bubbles just appear along the edges of the saucepan. Pour the cream over the chocolate and let it sit for 1 minute, then stir continuously until a smooth chocolate ganache forms. Pour it over the pumpkin layer then stick the tart in the freezer for 30 minutes. I find it easier to layer the marshmallow on top when it’s super cold.
To make the topping, combine the egg whites, sugar and cream of tartar in a heat-proof bowl, preferably the bowl of your electric stand mixer. Place over top of a double-boiler that contains simmering water, and whisk constantly for 3-4 minutes until the sugar has dissolved and the egg whites are slightly warm. Immediately remove the bowl and place it on your stand mixer with the whisk attachment, beating slow at first and gradually increasing the speed to high. Beat for 6-7 minutes until glossy and thick, then beat in vanilla extract for another minute until combined. Spread the marshmallow on the tart with the back of a spoon. You can torch the frosting if you’d like, as well as cover the top in additional graham crumbs. I stored this in the fridge with a layer of plastic wrap loosely over top.