Line mini muffin tins with cupcake liners. Place approximately 2 teaspoons graham cracker crumb mixture into the bottom of each liner and pack crumbs into the bottom of the liner. Place 1 teaspoon of the chopped bittersweet chocolate on top of the graham cracker crumbs. Transfer the muffin tins to the oven and bake until the edges of the graham cracker mixture is golden, about 5 minutes. Set aside.
Whisk together cocoa and hot water until smooth. In another bowl, whisk together flour, baking soda, baking powder and salt.
Melt butter with sugar in a saucepan over medium low heat, stirring to combine. Remove from heat and pour into a mixing bowl. With an electric mixer on medium low-speed, beat until mixture has cooled, 4 to 5 minutes. Add eggs, one at a time, beating until each is incorporated, scraping down sides of bowl as needed. Add vanilla, and then cocoa mixture in two batches, alternating with the sour cream and beating until just combined after each.
Add batter to graham cracker crust, filling liners approximately three-quarters full. Push two mini marshmallows into the batter and bake cupcakes approximately 16 minutes, rotating halfway through, or until a cake tester inserted in the middle comes out clean. Transfer tis to a wire rack to cool 10 minutes. Turn out cupcakes onto racks and let cool completely. Cupcakes can be stored overnight at room temperature or frozen up to 2 months, in airtight containers.
Place egg whites, sugar, and cream of tartar in the heatproof bowl of an electric mixer. Set over saucepan with simmering water. Whisk constantly until sugar is dissolved and whites are warm to the touch, 3 to 4 minutes.
Transfer bowl to an electric mixer fitted with the whisk attachment and beat, starting on low-speed, gradually increasing to high until stiff, glossy peaks form, 5 to 7 minutes. Add vanilla and mix until combined. Use immediately.
Frost cupcakes and lightly brown if desired with a kitchen blow torch. Serve immediately or store in an airtight container up to 2 days.