Preheat the oven to 170°C.
Beat all cake ingredients in a bowl until smooth and creamy.
Divide the mixture between two 18cm tins and bake for 35 minutes.
Combine the curd sugar, zest, juice, and eggs in a bowl.
Add butter pieces and stir over simmering water for 20 minutes until thickened.
Turn cooked cakes onto a wire rack to cool completely.
Slice each cold cake horizontally and sandwich with thick layers of curd.
Prepare long strips of lemon zest for the icing decoration.
Stir lemon juice into sifted icing sugar until soft and runny.
Spread icing over the cake and garnish with lemon zest.