Chocolate Hazelnut Crunch Cake

Ingredients

• 1 cup (128g) all purpose flour, sifted
• 1/2 cup (45g) Dutch process unsweetened cocoa powder, sifted
• 2 tablespoons (20g) cornstarch
• 1 teaspoon baking powder
• 1/2 teaspoon baking soda
• 1/2 teaspoon fine sea salt
• 1 cup (200g) granulated sugar
• 1/3 cup (66g) packed brown sugar
• 1/3 cup (74ml) vegetable oil or neutral oil of your choice
• 1 large egg
• 1 large egg yolk
• 1 teaspoon pure vanilla extract
• 2/3 cup (152g) sour cream
• 2/3 cup (156ml) hot coffee or espresso
• 4 large egg whites
• 2/3 cup (132g) granulated sugar
• 1 1/2 cups (340g) unsalted butter cubed, room temperature
• 1/2 teaspoon pure vanilla extract
• 1/8 teaspoon fine sea salt
• 1/2 cup (148g) Nutella or any hazelnut cocoa spread
• 8 hazelnut chocolate candies, roughly chopped (I used Ferrero Rocher)
• 2 tablespoons chopped toasted hazelnuts