Make the cake: Preheat the oven to 350 degrees F. Lightly butter two 9-inch round cake pans and line the bottoms with parchment paper. Pulse the graham crackers into fine crumbs in a food processor. Add the flour, cornstarch, baking powder, baking soda, cinnamon and salt and pulse until well combined.
Beat the butter and both sugars in a large bowl with a mixer on medium-high speed until light and fluffy, 3 to 4 minutes. Beat in the eggs one at a time, beating well after each addition, then beat in the vanilla. Reduce the speed to low and mix in the pumpkin puree (the batter will look curdled). Beat in half of the flour mixture, then the buttermilk, then the remaining flour mixture. Divide the batter evenly between the prepared pans. Tap the bottoms of the pans against the counter to release any air bubbles.
Bake the cakes until a toothpick inserted into the centers comes out clean, 40 to 45 minutes. Transfer to a rack and let cool 10 minutes in the pans, then loosen the edges with a knife and invert the cakes onto the rack; discard the parchment and let cool completely.
Meanwhile, make the filling: Microwave the chocolate, marshmallows, cocoa powder and 1/3 cup heavy cream in a bowl in 30-second intervals, whisking occasionally, until melted and completely smooth. Stir in the vanilla. Let cool to room temperature.
Whisk the remaining 2/3 cup heavy cream in a medium bowl until stiff peaks form. Whisk one-third of the whipped cream into the chocolate-marshmallow mixture, then gently fold in the rest with a rubber spatula. Refrigerate until firm, about 1 hour.
Meanwhile, make the meringue: Whisk the egg whites, granulated sugar, cream of tartar and salt in a large heatproof bowl. Place over a saucepan of simmering water (do not let the bowl touch the water) and cook, whisking, until the mixture is warm and the sugar dissolves, 3 to 4 minutes. Remove the bowl from the pan. Add the vanilla to the bowl and beat with a hand mixer on medium-high speed until the mixture is cool and stiff glossy peaks form, about 12 minutes (or transfer to a stand mixer and beat 6 to 8 minutes).
Assemble the cake: Using a long serrated knife, split each cake in half horizontally to make 4 layers total. Place 1 layer on a cake stand or plate. Transfer 1 cup meringue to a large resealable plastic bag and snip a corner. Pipe a ring of meringue on the perimeter of the cake layer (the meringue will keep the filling from oozing out). Spread one-third of the chocolate filling inside the ring. Repeat with 2 more cake layers, then top with the final layer. Cover the top and sides of the cake with the remaining meringue, using the back of a spoon to make peaks. Brown the frosting with a kitchen torch. Refrigerate the cake for 1 hour before serving.