This cake, rich with hazelnuts, is named for Oscar II (1829-1907), king of Sweden and Norway.
Preheat oven to 425 degrees. Butter three 9-inch round cake pans, and line bottoms with parchment; butter parchment. Separate eggs, placing 12 egg whites in 1 bowl and 8 egg yolks in another (reserve remaining 4 yolks for another use). Beat egg whites with a mixer until stiff peaks form. Gently fold in hazelnuts and confectioners' sugar. Divide among pans, and bake until golden brown and a toothpick inserted into centers comes out clean, about 20 minutes. Let cakes cool in pans 10 minutes. Invert cakes onto wire racks, and let cool completely. (Cake layers can be made 1 day ahead, wrapped tightly in plastic wrap, and refrigerated.)
Lightly whisk egg yolks in a bowl set over a pot of simmering water. Whisk in cream, granulated sugar, and salt, whisking constantly until mixture is thickened and coats the back of a spoon. Remove from heat, and let cool in a bowl of ice water until cold (you should have 2 cups).
Melt butter (do not let it color), and add vanilla. Stir into egg-yolk mixture. Place 1 cake layer on a cake stand or serving plate, and spread with 1/2 cup glaze. Repeat with a second layer and another 1/2 cup glaze. Place third layer on top, and spread with remaining 1 1/2 cups glaze, letting it drip down sides of cake. Dust with confectioners' sugar, and garnish with almonds. Refrigerate until ready to serve.