Heat oven to 350°F (175°C). Grease a 9 inch round metal cake pan at least 2 inches deep and line its base with parchment. Whisk flour, baking powder and salt in a bowl.

Rub zest into sugar with clean fingertips to release its aroma. Whisk with eggs for 1–2 minutes until smoothly blended and slightly lighter. This is a simple whisked cake, not a separately whipped sponge.

Slowly whisk in olive oil, then milk, orange juice and vanilla. Scrape the sides and bottom so the mixture is uniform.

Add dry ingredients and fold gently just until no dry flour remains. Small lumps are preferable to prolonged beating. Pour into the prepared pan and level the surface.

Bake 40–45 minutes, checking from 35 minutes if your oven runs hot. The center should spring back lightly and a skewer should emerge with a few crumbs but no wet batter. If still wet, continue in 3–5 minute intervals. Tent loosely with foil if the top darkens too quickly.

Cool in the pan for 15 minutes. Run a thin knife around the edge, turn out carefully, remove parchment and cool on a rack for about 1 hour before slicing. Serve plain; no glaze or frosting is included.