Sift together twice: flour, baking powder and salt. Pour back into sifter.
In a large mixing bowl beat egg yolks and whole eggs with an electric mixer until thick and lemon coloured (about 5 minutes). Gradually add sugar beating after each addition. This should take about 10 minutes.
Fold in orange zest, orange juice and lemon essence. Sift dry ingredients into egg and sugar mixture and fold in. Do not stir or beat. Add boiling water and fold in quickly just until liquid is blended. Pour batter into one ungreased 25cm ring tin.
Bake for 60 to 65 minutes. Turn cake over in tin on a wire rack and let cake hang for 1 hour or until cool. Loosen cake sides from tin with a spatula and shake from tin. Dust top lightly with icing sugar.