Preheat the oven to 325 degrees F. Grease a 9-inch springform pan and line the bottom of the pan with parchment.
Finely grind the hazelnuts in a food processor until they reach a coarse, sandy consistency.
Either in the microwave or in a double boiler over hot water, melt the chocolate with the oil and butter, stirring together to form a homogenous mixture. Set aside in a warm place.
With a stand mixer fitted with the whisk attachment, beat the egg whites and salt to soft peaks and set aside. Switch to the paddle attachment and, in a separate bowl, beat the egg yolks, sugars, and vanilla extract on medium-high speed until
thick and creamy, about 5 minutes. Reduce the speed of the mixer to low and add the
chocolate and butter mixture, then the cocoa powder. Fold in the whipped egg whites until smooth.
Pour into a greased and parchment-lined 9-inch springform pan and bake for 45 to 50 minutes, just until a wooden skewer inserted into the center of the cake comes out clean.