For the cake: Preheat the oven to 350 degrees F. Grease and flour an 8-inch cake pan.
Sift together the cake flour, cocoa powder, baking powder and baking soda in a bowl. Cream the butter and sugar in a stand mixer fitted with the paddle attachment. Add the eggs, one at a time, mixing on high speed until well incorporated. Add the sour cream, hazelnut paste and oil; mix well to combine. Add the chocolate and vanilla, and mix on low speed until very well incorporated. Add the dry ingredients and mix to combine.
Spread the batter in the prepared cake pan. Bake until the cake springs back to the touch or a tester comes out clean, 25 to 30 minutes. Let cool in the pan on a wire rack for 10 minutes, then remove from the pan to cool completely.
For the hazelnut chocolate buttercream: Whisk the sugar and egg whites in a stand mixer bowl set over a hot water bath until the temperature reaches 140 degrees F. Transfer the bowl to the mixer and mix on high speed, using the whisk attachment, until stiff peaks form, 8 to 10 minutes. Add the butter, a few pieces at a time, and cream until well combined. Add the chocolate and hazelnut paste, and mix well.
For the salted caramel: Heat the sugar and glucose syrup in a pan over medium heat until the sugar melts and caramelizes. Remove from the heat and add the heavy cream, salt, and vanilla seeds if using; stir to combine. Return the pan to the heat and cook until the caramel comes to a boil. Remove from the heat and let cool to room temperature.
To assemble: Slice the cake horizontally into three layers, each about 1/3-inch thick. Place one layer on a cake circle or serving plate and drizzle the salted caramel over the top. Using an offset spatula, spread some buttercream over the cake layer. Repeat with the remaining layers. Spread the sides of the cake with buttercream and smooth the edges with the spatula. Refrigerate until the buttercream is slightly firm to the touch, about 30 minutes.
Recipe courtesy of Santosh Tiptur