Heat oven to 180°C. Grease a round cake tin, then line the base with parchment. Using electric beater, whisk the egg whites, cream of tartar and a pinch of salt until stiff but not dry. Add a tablespoon of sugar, then whisk until the mixture thickens again to make stiff peaks. Repeat with the rest of the sugar, spoon by spoon, until the mix resembles thick shiny shaving foam. Set aside.
Crack the eggs into a large mixing bowl, add the sugar, spoon by spoon, then whisk with electric beater until thick, light and creamy. Add the flour and baking powder alternately with the diluted milk, beginning and ending with flour. Add lemon rind and beat until well blended.
Gently fold a third of the meringue into the cake mixture to loosen it. Fold in the remaining egg white mixture until even.
Slowly pour the batter into the tin, then use a spatula to get every last bit from the bowl. Bake for 30-35 minutes, or until the cake is risen and set on top. Cool the cake in its tin on a rack.
Carefully remove the cake from its tin into a serving plate, slice the cake horizontally, then spread margarine in one disk and sprinkle icing sugar. Put the two disks together, and spread margarine over the top and sides of the cake. Dust with icing sugar.
Leave to set for a few minutes, and either chill for a few hours, or serve straight away. Share and enjoy.
