In the bowl of an electric mixer fitted with the whisk attachment, beat the butter and the granulated sugar for 2 minutes. Add the oil and continue beating for a minute.

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

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

Use half of the batter to fill each cake tin, about 7oz in each, then drizzle nutella on top of the batter. Place the rest of the batter on top of the nutella swirl and repeat with another drizzle. Use a butter knife to swirl the batter and nutella back and forth.

Bake for 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. Invert the cakes onto a wire rack or tea towel and allow them to cool completely before frosting.

Place the room temperature 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.

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.

To frost the cake, place the first cake layer on a flat surface and use 1/4 of the buttercream to frost the first layer. Drizzle more Nutella on top of the buttercream and repeat with the second layer.

Place the third layer of cake on top and smooth any buttercream that squished out on the edges of the cake. Refrigerate the cake for at least 30 minutes, or until the cake feels firm enough to frost (make sure it's not wobbly)

For the crumb coat, place 1/4 of the buttercream on top of the cake and use an offset spatula to smoothly spread it evenly throughout. It doesn't have to be perfect. Refrigerate the cake for at least another 15-30 minutes, or until the frosting feels set.

Place the rest of the buttercream on top of the cake, use an offset spatula to spread the frosting around and then use a turn table and a cake scraper to create a smooth finish on the cake.

Use a piping bag with a tiny circle tip to drizzle nutella on top of the the cake. Top with chopped hazelnuts, sea salt flakes and gold foil.