Ultimate Nutella Cupcakes

INGREDIENTS

For the chocolate cupcakes::

4 oz

bittersweet chocolate, finely chopped

1/3 cup

Dutch-process cocoa

1 tsp

instant espresso powder

1/2 cup

boiling water

3/4 cup

plus 2 tablespoons (113 grams) all-purpose flour

1/2 cup

granulated sugar

1/4 cup

packed light brown sugar

1/2 tsp

salt

1/2 tsp

baking soda

1/2 cup

sour cream, at room temperature

1/4 cup

vegetable oil

large eggs plus one egg yolk, at room temperature

2 tsp

white vinegar

1 tsp

vanilla extract

For the filling::

1/2 cup

Nutella

For the Nutella buttercream::

2 cups

powdered sugar, sifted

8 oz

unsalted butter, at room temperature

2/3 cup

Nutella

1/4 tsp

fine salt

1 tsp

vanilla extract

Ferrero rocher candies, for topping