1.Preheat oven to 180°C (160°C fan-forced).
2.Grease a 19cm x 29cm lamington pan.
3.Sift flour into a medium bowl, rub in butter. Add enough milk to make ingredients cling together. Knead gently on a lightly floured surface until smooth. Roll dough large enough to cover base of pan. Spread with jam, sprinkle with currants; spread with topping. Bake for about 25 minutes. Cool on wire rack. When cold, spread with icing, cut when set.
4.To make topping: Beat butter, essence and sugar in a small bowl with an electric mixer until light and fluffy; beat in eggs, one at a time, until combined. Stir in flour and milk.
5.To make lemon icing: Sift icing sugar into a small bowl, stir in butter and enough lemon juice to make icing spreadable.
This keeps for 4 days.Note