Sift together twice: flour, baking powder and salt. Pour back into sieve.
In a large mixing bowl beat egg yolks and whole egg with an electric mixer until thick and lemon coloured (about 5 minutes). Gradually add sugar while beating. This should take about 10 minutes.
Fold lemon zest and juice into the egg mixture. Sift in dry ingredients and fold gently. Do not stir or beat. Add boiling water from the kettle and fold in quickly, just until liquid is blended. Pour cake mixture into an ungreased 25cm tube cake tin.
Bake at 170 C / Gas 3 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 loose.
Instead of the 25cm tube tin, you can use three 23cm round cake tins or two 23x12cm loaf tins. Adjust baking time as needed.
Dust top lightly with icing sugar or ice with your favourite buttercream or lemon icing.