Cream together the butter and powdered (icing) sugar, then add the flour. Press into a sponge roll tin (9x13") lined with baking paper, and bake for 15 mins at 340°F (170°C).
If you have a food processor, peel the lemon with a potato peeler, then chop the rind very finely with the sugar, using the metal chopping blade.
Add the eggs and process again until well mixed, then add the remaining ingredients and process until smooth.
Pour this mixture over the partly cooked base, and bake at 340°F (170°C) for 30 minutes. Dust with powdered sugar and cut into neat rectangles while it is warm.