1.Grease a 19cm x 29cm lamington pan.
2.Sift flour into bowl; rub in butter. Add enough water to make ingredients cling together. Knead gently on lightly floured surface until smooth. Cover pastry; refrigerate 30 minutes.
3.Meanwhile prepare topping. Beat egg white in small bowl until stiff, beat in sugar, fold in cake crumbs, sifted cocoa, peanuts and essence; mix well.
4.Roll out pastry large enough to cover base of prepared pan; prick well with fork. Bake in moderate oven for about 15 minutes; cool 10 minutes; spread evenly with jam, then topping. Brush surface evenly with water. Bake in moderate oven for about 20 minutes. Cool in pan before cutting.