Make the cupcakes:
In a medium heatproof bowl add the chocolate, cocoa powder, and espresso powder. Pour hot water over mixture. Cover and let stand for 5 minutes. Whisk mixture gently until smooth then set aside to cool.
Meanwhile, preheat the oven to 350°F. Line a standard 12-cup muffin tin with foil or paper liners.
In a small bowl, whisk together the flour, sugars, salt, and baking soda.
Whisk sour cream, oil, eggs, egg yolk, vinegar, and vanilla into the cooled chocolate mixture until smooth. Stir in the flour mixture until smooth, being careful not to overmix.
Using a spring-loaded scoop, divide the batter evenly among prepared muffin cups. Bake until cupcakes are set and firm to the touch, about 20 minutes. Let cupcakes cool in pan on a wire rack for 10 minutes before removing from pan to cool completely, about 1 hour. Cupcakes can be stored in an airtight container at room temperature for up to 1 day.
Fill the cupcakes:
Use an apple corer or small paring knife to remove the center of each cupcake, being careful not to remove the bottom.
Place the Nutella in a ziptop bag and snip a hole in one corner. Gently squeeze 2 teaspoons of Nutella into each cupcake.
Make the frosting:
In the bowl of a stand mixer fitted with the paddle attachment, mix together the sugar and butter. Mix on low speed until well blended and then increase the speed to medium and beat for another 3 minutes. Add the Nutella, salt, and vanilla and beat on medium for 1 minute, or until well combined.
Fit a piping bag with a decorative tip and frost all the cupcakes. Top with a Ferrero Rocher candy, if desired. Serve or store in an airtight container at room temperature for up to 1 day, or in the fridge for up to 3 days.