Begin by bringing all your ingredients to room temperature. Preheat the oven to 350F/177C convection (325F/163C conventional) and grease and line three 8 inch cake pans.

In a medium bowl, whisk together the flour, cocoa powder, baking powder and salt until they're evenly distributed.

Place the butter and the granulated sugar in a large bowl and use an electric mixer with the whisk attachment to beat them for 2 minutes.

Add the oil and beat at full speed for another 1-2 minutes.

Add the eggs, one at a time, beating on medium speed until each egg is fully incorporated. Add the vanilla and sour cream and beat just until it's combined.

Add half of the dry ingredients to the batter, turn the mixer on low speed and then pour in the milk, followed by the rest of the dry ingredients.

Distribute the batter evenly among the three cake pans, about 18oz in each pan and bake for 30-35 minutes. When the cake is fully baked it will release itself from the edge of the pan and it will spring back if you press on the center of it.

Allow the cake to cool in the pan for 5 minutes and then invert them onto a tea towel or cooling rack. Then allow them to cool completely before frosting.

While the cake is baking, place the hazelnuts in a food processor and grind them until they're small crumbles.

Place the ground hazelnuts in a short, wide bowl or on a baking sheet. Set aside.

Place the sugar and water in a sauce pan over medium heat. Stir intermittently until it is fully dissolved and the color is like honey or a little darker.

Pour the sugar evenly over the hazelnuts and stir quickly before it cools and hardens.

Return the nuts with the sugar to the food processor and pulse until there are no more large chunks. 

Heat up the heavy cream and remove it just before boiling. SLOWLY pour over the chocolate while stirring.

Stir until the chocolate is silky smooth, then add 1 cup of the hazelnut praline.

Set aside and allow the ganache to cool.

Place the butter in the bowl of an electric mixer and use the whisk attachment to beat the butter on medium-high speed for 3-4 minutes, until the color turns to a pale yellow, scraping the edge of the bowl half way through.

Add the Nutella and mix on medium-high speed. Scrape the edge of the bowl and beat again until the butter and Nutella are fully combined and smooth.

Add the cocoa powder and vanilla extract and mix until they're fully combined.

If the buttercream feels too soft, refrigerate for 10 minutes and then use a spatula to scrape the sides and mix it again. Repeat until the buttercream feels firm enough to frost the cake.

Place the frosting in a piping bag fit with a round tip.

Spread a thin layer of frosting on the first layer of cake and then pipe a boarder of frosting around the edge.

Fill the border in with a layer of cooled praline ganache. Refrigerate the cake for about 15-20 minutes so the buttercream can firm up before stacking the next layer.

Repeat with the second layer of cake. Refrigerate and then place the third layer on top.

Refrigerate the cake for 30 minutes minutes, or until it feels sturdy enough to frost the outside.

Top with ferrero rocher chocolates and more hazelnut praline.