How to cook
Mix the flour with the baking powder and salt. Preheat the oven to about 347°F (175 °C). Beat the butter and sugar until blanched for about 5 minutes. One by one, break the eggs, while beating them with a mixer continuously.
Add the flour is in 2 lots and stir until homogeneous. Throw in the orange zest and transfer the mixture into a lightly greased and floured oven dish or cake form.
Put the cake to bake for half an hour, checking to see if it's ready with a toothpick.