• Place an oven rack in the middle position. Preheat oven to 375 degrees F. Spray a 9-by-3-inch round cake pan with vegetable spray; set aside.

Make the sponge cake:

• Grind the hazelnuts in a food processor with 1/4 cup sugar. In a medium bowl, sift together flour and cornstarch and then add hazelnut mixture; set aside.
• In the bowl of an electric mixer fitted with the whisk attachment, beat the whole eggs, the yolks, remaining 1/2 cup sugar, salt, and almond extract on high speed until thick and pale yellow in color, and mixture holds a thick ribbon when whisk is lifted from bowl, 7 to 8 minutes. Remove bowl from mixer; gently and quickly fold in flour mixture. Gradually drizzle in oil, while folding.
• Pour batter into prepared cake pan and bake until a tester comes out clean when inserted into the center of the cake, about 25 to 30 minutes. Remove from oven and turn cake out immediately onto a wire rack. Let cool for at least 1 hour.

Make the chocolate mousse:

• In a medium saucepan, whisk together egg yolks, 1 tablespoon sugar, and 1/2 cup heavy cream. Cook over medium-low heat, stirring, until mixture coats back of spoon, 3 to 4 minutes (do not boil). Remove from heat; whisk in melted chocolate and vanilla. Chill until cool.
• With an electric mixer, beat remaining ½ cup heavy cream with remaining 1 tablespoon sugar until stiff peaks form. Stir 1/3 of whipped cream into cooled custard mixture, then gently fold in the rest with a rubber spatula.
• Spread the chocolate mousse over the top of the cake. Transfer in the fridge for at least 15 minutes to set.