Roll out pastry and line a 28cm loose-bottomed flan tin. Chill for 20 minutes. Preheat oven to 200 degrees celcius.

Line the pastry case with aluminium and fill with baking weights, dried beans or chick peas. Remove from the oven, remove weights and foil and brush base of tart with apricot jam. Reset oven to 180 degrees celcius.

For filling, process walnuts until quite fine.

Beat together caster sugar and egg yolks until thick and pale. Add ground walnuts, softened butter, cream and rum.

In a clean dry bowl whisk egg whites until soft peaks form, then gently fold into nut mixture.

Pour filling into tart shell. Bake for 45 minutes until filling is golden and feels springy when touched lightly with the palm of your hand. Remove tart from oven and leave to cool before icing.

Meanwhile, lightly toast walnut halves on a baking tray in turned-off oven.

For icing, place a bowl over a saucepan of hot water over medium heat (the bottom of the bowl must not touch the water). Add chocolate and butter and allow to melt..Stir until smooth.

Pour over cold tart, tilting to mixture flows evenly. Mark each portion with a walnut half and allow icing to cool before slicing.