Heat oven to 350 degrees F/180 degrees C. Rub a 9-inch/23-cm springform pan with olive oil and line the bottom with a parchment paper.
Beat the egg yolks and sugar until thick, pale and ribbony. Mix in the olive oil, lemon juice and zest and the orange zest. Add the flour and stir to combine. Beat the egg whites with the salt to soft peaks and gently fold into the batter. Fold in half the grapes. Pour into prepared pan and bake for 20 minutes. Sprinkle the remaining grapes onto the top of the cake and continue to bake until a toothpick comes out clean when inserted into the center of the cake, about 25 minutes. Cool on a rack, then unmold. Brush the top generously with extra-virgin olive oil and sprinkle some dark brown sugar over the cake.