Preheat your oven to 350°F/175°C and place 14 cupcake liners in your cupcake pans.
In a large bowl, whisk together the flour, sugar, light brown sugar, cocoa powder, baking powder, baking soda and salt. Set aside.
1 cup + 2 tablespoons all purpose flour, ½ cup granulated sugar, ½ cup light brown sugar, ½ cup cocoa powder, ½ teaspoon baking powder, 1 teaspoon baking soda, ½ teaspoon kosher salt
In a mug or small cup, mix together the espresso powder and hot water until the espresso powdered has dissolved. Set aside.
1 teaspoon espresso powder, ½ cup hot water
In a large jug, whisk the eggs, oil, sour cream, and vanilla until smooth.
2 large eggs , ⅓ cup vegetable oil, 1 teaspoon vanilla extract, ½ cup sour cream
Pour the wet mixture into the flour, stirring until it is just about smooth - we don't want to over mix the batter! Then pour in the hot coffee and mix until smooth.
Fill the cupcake liners so they're ¾ full and bake for 18-20 minutes. A toothpick inserted should come out clean.
Allow the cupcakes to cool for 15 minutes in the tray, then take them out and place on a cooling rack.
Chocolate Filling
Place the chopped chocolate in a large bowl. Pour the heavy cream into a small saucepan and heat over medium-low heat until steaming - you don't want it to boil.
½ cup heavy cream
Pour over the steaming heavy cream over the chopped chocolate, let sit for 5 minutes, then stir until smooth. Let sit until hardened.
4 oz semisweet chocolate
Once your cupcakes have cooled completely, use a cupcake corer or the base of a large piping tip to core each cupcake. Set the cupcakes aside.
Scoop out chocolate ganache from the medium bowl into a piping bag with a round tip, and fill each cupcake with ganache. You can also use a spoon to do this but I found using the piping bag was much easier! If the ganache has gotten too hard, place in the microwave in 5 second increments until it's scoopable.
Marshmallow Frosting
Place the egg whites, sugar, salt, and cream of tartar in a heatproof bowl - I prefer using a metal bowl for this.
4 large egg whites , ¾ cup granulated sugar, ½ teaspoon cream of tartar, pinch salt
Fill a saucepan with an inch or two of water on the stove and place over medium-low heat, so the water begins to simmer. Place the bowl on top of the saucepan, making sure the bottom of the bowl doesn't touch the water.
Whisk constantly until the sugar dissolves. It'll go from thick to frothy. To test, I like to (carefully!) lift the whisk up and rub the mixture between my fingers. You shouldn't feel any of the sugar.
Take the bowl off the double boiler and place in your stand mixer, fitted with the whisk attachment (or use a handheld mixer). Add the vanilla.
1 teaspoon vanilla extract
Beat on high speed for 5 minutes, or until you get stiff and glossy peaks.
Frost your cupcakes. Optional, but use a kitchen torch to toast the frosting and enjoy!
Notes
My recipes have been developed and tested in metric grams. I highly recommend using a weighing scale and measuring out your ingredients the metric way. I have provided a conversion to US cups but please note that I haven't tested the recipe with this method. Ingredients: I make sure that all of my ingredients are at room temperature before I start the recipe, so pull out all of the ingredients before you start baking.Salt: If you don't have kosher salt, halve the amount of salt called for in the recipe. 1 teaspoon kosher salt = ½ teaspoon fine salt. Please note, I'm using Diamond Crystal kosher salt and not Morton salt which is too salty! Cocoa Powder: I'm using natural unsweetened cocoa powder (not sure what you have? Check the ingredients. If it says 'acidity regulator' or mentions 'alkali', then it's dutch processed). Storage: You can store the cupcakes in the fridge for 3-4 days. I recommend letting it come to room temperature before enjoying! I do find that because of the meringue frosting, the cupcakes are best enjoyed on the day of frosting.