Preheat the oven to 350 degrees Fahrenheit (180 C). Spray a 9×13 pan with nonstick cooking spray.
Sift all dry ingredients into the large bowl of an electric mixer fitted with a paddle attachment. Add the rest of the ingredients and mix on low speed until the drys are moistened. Turn the speed to medium and mix the batter for 2 minutes. Scrape down the bottom and sides of the bowl, and mix for 30 seconds more until your batter is homogenous and smooth. It will be thin.
Pour the cake batter into the prepared pan and bake it at 350 until the top springs back when pressed gently and a toothpick inserted into the center comes out clean, about 25 minutes. Let it cool completely. Once cool, cut the cake into small 1-inch cubes.
To Make the Mousse:
Make sure you start with hazelnuts that have their skins removed and that are toasted golden brown—this will ensure your mousse has the best flavor. Place the nuts in the bowl of a food processor and turn the processor on. When they have turned to powder, gradually stream in the hazelnut oil and continue processing the nuts. Stop the processor periodically to scrape down the bottom and sides. Continue to process the nuts until they have turned into a very fluid butter with a smooth texture. Depending on your processor this could take 3-8 minutes. If the nut butter has become warm, let it cool to room temperature.
Whip the heavy cream until stiff peaks form, then transfer it from the mixing bowl to another bowl. In the same mixing bowl, combine the hazelnut butter and the powdered sugar and mix on medium speed until well-incorporated. (It might be quite stiff.) Add the vanilla extract, salt, and mascarpone cheese. Beat on medium speed until everything is well-combined. Mascarpone breaks easily, so don’t overwhip the mousse or leave the mixer running unattended.
Fold in 1/3 of the whipped cream to lighten the hazelnut mixture, then add the rest and gently fold everything together.
To Make the Whipped Ganache:
Place the chopped chocolate in a medium bowl, and pour the heavy cream into a small saucepan. Heat the cream until it comes to a simmer and bubbles appear around the edges. Pour the hot cream into the chopped chocolate and let it sit and soften the chocolate for one minute.
Whisk the cream and chocolate together until the mixture is smooth and shiny. Press a layer of cling wrap on top of the chocolate. If you have the time, leave the ganache at room temperature for an hour or two to cool down and thicken. If not, speed up the process by putting it in the refrigerator for 30-45 minutes, until it is the texture of peanut butter.
Once cooled, whip it in a mixer using a whisk attachment until it is lightened and thickened, like frosting.
To Assemble & Decorate the Trifle:
Whip the 1 cup of heavy cream with the powdered sugar and vanilla until stiff peaks form.
Scatter a layer of cake cubes in the bottom of a large glass bowl. Spoon half of the hazelnut mousse on top and spread it evenly across the cake. Pour half of the whipped ganache on top of that, and spread it out, then top it with a layer of toasted hazelnuts. Repeat the layers, adding cake, hazelnut mousse, whipped ganache, and chopped hazelnuts. Finish with a layer of cake, then top the cake with the whipped cream. You can either spread it on, or put it in a pastry bag and pipe rosettes or swirls on top.
Decorate the top of the trifle with additional hazelnuts, chocolate-covered nuts, and chocolate curls.