Preheat the oven to 350°F (180°C). Line a 9-inch square baking pan* with parchment paper.

In a large bowl, whisk together the flour, cocoa powder, cornstarch, baking powder, baking soda, and salt until well combined. Stir in the granulated sugar and brown sugar.

n a medium-size bowl, whisk together the oil, egg, egg yolk, vanilla extract and sour cream. Add this mixture to the dry ingredients. Beat until combined, then gradually mix in the hot coffee until the batter is smooth. Transfer the batter to the baking pan.

Bake until a toothpick inserted into the center comes out mostly clean or with a few moist crumbs attached, about 18-23 minutes. Cool in the pan for 10 minutes, then remove the cake and carefully transfer it to a cooling rack. Frost the cooled cake. Top with the chopped chocolates and hazelnuts right before serving.

Combine the egg whites and sugar in a large heatproof bowl set over a pan of gently simmering water, making sure the water doesn’t touch the bottom of the bowl. Whisk constantly until the sugar is dissolved reaches temperature of 160ºF (71°C)


Transfer the mixture to the bowl of a stand mixer with a whisk attachment. Whisk on low speed until frothy, gradually increasing to medium-high speed. Whisk until glossy, stiff peaks form and the mixture reaches room temperature.


Turn the mixer to medium-low speed. Add butter, a few pieces at a time, allowing the butter to fully incorporate before adding more. Turn the mixer up to medium speed. If the mixture looks curdled, keep mixing and it will correct itself. 


When the buttercream is smooth and well combined, switch to a paddle attachment. On low speed, mix in the vanilla extract, salt. Gradually mix in the Nutella. Continue mixing on low speed for a few minutes until smooth.