Grease a large baking sheet lightly and sprinkle with water. Melt the butter in a medium saucepan with the water. Bring to the boil, then remove from the heat.
Add the sifted flour and beat until the mix forms a ball. Allow to cool for 15 minutes, then gradually beat in the eggs until smooth and glossy. Add the vanilla essence.
Place 12 equal spoonfuls of the mix, spaced well apart, on the sheet and bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 25 minutes or until well risen and golden. Slit around the middle of each and return to the oven for 3 minutes. Transfer to a rack to cool.
Make the crème pâtissière. Slit the vanilla pod and scrape out the seeds. Put milk and double cream in a pan and add the seeds and pod. Bring almost to the boil, then leave to stand for 20 minutes. Beat together the yolks, caster sugar and plain flour. Remove the vanilla pod, reheat the milk mixture, then gradually whisk into the yolks. Pour back into the pan and cook gently for 4–5 minutes, stirring until very thick. Turn into a bowl, cover the surface with clingfilm and leave to cool.
Open out each puff and divide the sliced strawberries among them. Pile the crème pâtissière on top and push the puffs back together so the strawberries and crème pâtissière still show around the centre. Dust with icing sugar and store in a cool place until ready to serve.