Roll out pastry to fit an 20 cm pie plate. Flute edges and bake "blind" that is, bake without a filling. To preserve shape of pie, fit a circle of greaseproof paper into pastry-lined dish and fill with dried beans. Remove paper and beans for last 5 minutes of baking time about 25 minutes in all.
Meanwhile, make the cream filling.
Mix sugar, cornflour and salt together in a saucepan. Add milk gradually. Stir until mixture comes to the boil and for 1 minute longer. Stir half of the mixture into the slightly beaten egg yolks. Return to the pan and boil 1 minute. Remove from heat. Blend in butter and vanilla, and leave to cool. Slice bananas into base of cooled pastry shell. Cover with cream filling. Chill. Before serving, whip double cream and pipe or spoon round edge of pie.