Preheat your oven to 350°F (175°C) and line a cupcake baking tray with cupcake liners.
In a large mixing bowl, whisk together the flour, sugar, cocoa powder, baking soda, and salt.
In a separate bowl, mix the buttermilk, vegetable oil, eggs, vanilla extract, red food coloring, and white vinegar until well combined.
Gradually add the wet ingredients to the dry ingredients, mixing until just combined. Do not overmix.
Divide the batter evenly among the cupcake liners, filling each about two-thirds full.
Bake in the preheated oven for 20 minutes, or until a toothpick inserted in the center comes out clean.
Allow the cupcakes to cool in the tray for 5 minutes, then transfer to a cooling rack to cool completely.
While the cupcakes cool, prepare the marshmallow frosting. In a saucepan, combine sugar, water, and cream of tartar over medium heat. Stir until the sugar dissolves.
Once the sugar has dissolved, increase the heat and let the mixture boil without stirring until it reaches the soft-ball stage (240°F or 115°C).
While the sugar mixture is boiling, beat the egg whites and salt in a mixing bowl until stiff peaks form.
Carefully pour the hot sugar mixture into the egg whites while beating continuously. Beat for about 5-7 minutes until the frosting is glossy and holds stiff peaks.
Add the vanilla extract and mix until well incorporated.
Once the cupcakes are completely cool, frost them using a piping bag or a spatula with the marshmallow frosting.
Optionally, you can use a kitchen torch to lightly toast the frosting for a s'mores effect.