1. Preheat the oven to 350°F. Grease the sides and bottom of a 9-inch cake pan with olive oil, and line the base with a circle of baking parchment.
2. Combine the rice flour, potato starch, baking powder, and salt, and set aside.
3. Combine the olive oil, rum, orange juice, and orange zest.
4. Beat the eggs well, using an electric mixer. Slowly add the sugar and beat until very thick and light, about 5 minutes. Fold in the olive oil-citrus mixture while pouring it around the edges of the bowl.
5. Sift the flour mixture over the batter, and fold in. Fold in the chopped walnuts. Transfer the batter to the cake pan, and bake until golden and an inserted toothpick comes out clean, about 30 minutes. Let cool in the pan for 10 minutes. Loosen the sides of the cake with a knife blade, and turn out onto a cooling rack. Peel off the parchment paper, turn the cake right side up, and let cool completely. Sift confectioners’ sugar over the cake before serving.