Preheat the oven to 330ºF (165ºC). Line an 8.5-inch by 4.5-inch (22-cm by 11-cm) loaf pan with a sheet of parchment paper long enough to hang over the sides so it can act as a sling to remove the finished cake.
Add the eggs, sugar & extract to a mixing bowl, and beat until extremely light, airy & tripled in volume.
While the eggs & sugar are whipping, combine the flour and zest in a bowl. Whisk to blend.
Scatter half the flour over the egg mixture, and use a wooden spoon or spatula to gently fold it in. Repeat with the remaining flour.
Spoon the batter into the prepared pan. Bake 30 minutes, until nicely golden, and the top springs back when gently tapped with your finger.
Set the pan, upside down, on a wire rack, and let cool completely before cutting.