How to Make It
Preheat the oven to 350°. Spread the walnuts in a cake pan and bake until fragrant and lightly toasted, about 8 minutes. Transfer the walnuts to a work surface and let cool, then coarsely chop.
Spray a 9-inch springform pan with nonstick spray and dust with cocoa powder. In a large bowl, whisk the flour with the baking powder and salt. In a medium saucepan, stir all but 2 tablespoons of the superfine sugar with the 3/4 cup of cocoa powder. Whisk in the water in a steady stream until incorporated. Bring the cocoa mixture to a boil over moderate heat, whisking occasionally. Remove from the heat and whisk in the olive oil.
In a medium bowl, beat the egg yolks lightly with a fork to break them up. Beat a large spoonful of the cocoa mixture into the egg yolks, then scrape the yolk mixture into the saucepan and whisk until no streaks remain. Using a rubber spatula, stir the chocolate mixture into the dry ingredients, then fold in the chopped walnuts.
In a medium bowl, using a hand mixer, beat the egg whites until soft peaks form. Gradually add the remaining 2 tablespoons of superfine sugar and beat until the egg whites are thick and glossy, about 5 minutes. Fold the beaten whites into the batter in 3 additions.
Spread the batter in the prepared pan and bake for about 1 hour, until a skewer inserted in the center comes out with a few moist crumbs attached. Transfer the cake to a rack and let cool completely before unmolding. Sift confectioners' sugar over the top before serving.