Preheat oven to 190°C (170°C for fan assisted)/375°F/Gas Mark 5.
Halve the pastry, roll out to a thickness of 3mm and rest for 5 minutes. Using a 10cm fluted cutter, cut circles of pastry and transfer to non-stick baking sheets. Prick the bases and bake in the oven for 12 minutes until golden brown. Then cool on a rack.
To make the chocolate custard, mix the custard powder with the cocoa powder, pour on the boiling water and whisk until smooth. Allow to cool completely (refrigerate if time allows).
To finish, whip up the cream topping with the milk and transfer to a piping bag fitted with nozzle.
When cool, spread each pastry round with a teaspoon of cold chocolate custard. Slice the bananas and place three slices on each portion. Finish with a swirl of cream and dust with cocoa powder.