Preheat the oven to 350°F (175°C). Line a 12-cup muffin tin with cupcake liners.

Prepare the crust: Mix graham cracker crumbs, sugar, and melted butter in a bowl. Press about 1 tablespoon of this mixture into the bottom of each cupcake liner. Bake for 5 minutes and let cool.

Make the chocolate batter: In a bowl, whisk flour, cocoa powder, baking powder, baking soda, and salt. In another bowl, whisk eggs, sugars, oil, buttermilk, and vanilla. Combine wet and dry ingredients until smooth.

Fill liners: Divide batter evenly among liners (3/4 full) over the cooled crust.

Bake cupcakes for 18–20 minutes, or until a toothpick inserted in the center comes out clean. Cool completely before frosting.

Prepare marshmallow frosting: In a heatproof bowl, combine egg whites, sugar, and cream of tartar. Place over a pot of simmering water and whisk constantly for about 4 minutes until sugar dissolves and mixture is warm.

Whip frosting: Remove from heat and beat with an electric mixer on high speed until stiff peaks form. Add vanilla extract and mix briefly.

Frost cupcakes: Pipe or spoon the marshmallow frosting onto cooled cupcakes.

Toast frosting: Use a kitchen torch to lightly toast the tops until golden brown.