Preheat oven to 350 degrees. Spray a 8-inch round cake pan with non-stick cooking spray and line the bottom with parchment paper.
Into a food processor, combine all dry ingredients (flour, walnuts, baking powder, baking soda and salt). Process for one minute and 30 seconds, until finely ground. Transfer mixture to a bowl and keep aside.
Into the same (now empty) processor, add butter, sugar and orange zest (from one orange). Process for 45 seconds. Add both eggs and process for few seconds until smooth (scrape the sides of the processor to mix in any butter sticking to the sides).
Add half the ground flour mixture, process for 10 seconds, then orange juice, process for 10 seconds and then finish off with remaining flour mixture and process for additional 10 seconds, until combined.
Transfer batter into the prepared pan, spread evenly and sprinkle orange sugar on top.
Bake in the middle rack of the oven for 36-41 minutes, until a tooth pick inserted in the middle comes clean.
Place pan on a wire rack to cool (we usually start digging in while still warm). YUM.