Preheat oven to 350°F and line a muffin pan with cupcake liners
Make the Chocolate Cupcakes
Sieve all the dry ingredients into a large mixing bowl
Add the eggs, oil, milk and vanilla and beat for a couple of minutes until combined
Add hot water and mix together. The batter will be fairly liquid
Spoon the batter into the cupcake liners and bake for 18 minutes or until a skewer comes out clean
Make the Melty Chocolate Filling
Chop your chocolate into small pieces and place in a bowl
Heat the cream in a small saucepan until it is scalding hot (little bubbles appear on the surface, this is the stage before is starts to boil)
Remove from the heat and pour the cream into the chopped chocolate, let sit for 30 seconds
Stir the mixture together until smooth and silky. Let cool
Make the Marshmallow Frosting
Whip the egg whites and cream of tartar on high until soft peaks form
Separate ½ a cup of the sugar into a bowl and slowly add it to the egg whites, one tablespoon at a time, whipping in between each addition
Add the remaining sugar and the water to a saucepan and heat to the soft ball stage/ 240°F (see description above if you don't have a candy thermometer)
While still whipping, slowly pour the sugar syrup into the egg whites in a slow, steady stream
Add the vanilla
Continue to whip on high until the mixture has completely cooled down. Your bowl should no longer feel warm to the touch once it's ready
Your meringue should be thick and glossy
Construct your S'mores Cupcakes
Once the cupcakes have completely cooled, core out the middle of the cupcake using a cupcake corer or teaspoon
Fill the cavity with chocolate filling
Pipe or spread the marshmallow frosting onto the cupcakes
If you want to, you can blowtorch the marshmallow frosting for a 'toasted marshmallow' look
Drizzle remaining chocolate filling over the top of the marshmallow frosting and sprinkle with some graham crackers crumbs