Preheat the oven to 375°F. Lightly butter and flour an 8-inch round cake pan. Using an electric mixer fitted with the paddle attachment, or by hand with a wooden spoon, beat the egg yolks and granulated sugar together until light and fluffy, about 4 minutes. Stir in the lemon zest.

In another bowl, using the whisk attachment, beat the egg whites until they hold stiff peaks. Fold one-quarter of the whites into the egg-yolk mixture to lighten it. Add the ground walnuts, stirring until thoroughly blended. Carefully fold in the remaining egg whites to maintain a light texture.

Pour the batter into the prepared pan and bake for 55 to 60 minutes or until firm and golden brown. Cool on a rack before removing the cake from the pan.

Cut into wedges and serve garnished with confectioners’ sugar, cocoa powder, and fresh berries.