Grease about 18 individual, fairly shallow patty (muffin) tins.
Cream the butter and sugar together until lighter in colour, add the vanilla essence and egg yolk.
Work the flour into the mixture.
Roll out the pastry thinly on a floured board and use to line the patty tins.
Prick the bases with a fork, then bake at 375°F, Gas Mark 5 for 10-15 minutes until golden brown.
Remove from the tins and cool on a wire rack.
Melt the chocolate, butter and cream in a pan over gentle heat, stirring continuously.
Remove from the heat, add rum and allow to cool completely, beating occasionally until thick.
Divide between the pastry cases.
Add enough water to the icing (confectioners') sugar to make a thick glace icing.
Spoon over the chocolate mixture and decorate with a chocolate curl or piece of flaked chocolate bar.
Leave to set.