Preheat the oven to 220C/425F/Gas 7. Line a baking tray with baking parchment and draw onto it eight circles 5cm/2in wide and another eight circles 2.5cm/1in wide. Put the butter in a heavy-based saucepan with 150ml/5fl oz of water and heat over a medium heat until the butter melts. Bring the mixture to the boil and then immediately remove from the heat.
Quickly tip in the flour. Stir vigorously with a wooden spoon until the mixture forms a soft ball. Return to the heat and cook over a low heat for 3-5 minutes, stirring constantly.
Remove from the heat and leave to cool slightly. Gradually add the eggs, beating well between each addition to form a smooth, shiny paste. Spoon the mixture into a piping bag fitted with a 1.5cm/ ½in plain nozzle.
Pipe round discs onto the baking tray in the marked circles and, using a damp finger, smooth over the top of each disc. Bake in the centre of the oven for 10 minutes. Reduce the oven temperature to 190C/375F/Gas 5 and cook for a further 10-15 minutes. Remove the choux buns from the oven and pierce each bun with a skewer to allow the steam to escape. Return to the oven for 4-5 minutes to dry out. Remove from the oven and leave to cool on a wire rack.
For the crème pâtissière filling, pour the milk and vanilla seeds into a heavy-based pan and bring gradually to the boil. Remove from the heat and leave to cool for 30 seconds. Meanwhile, in a medium bowl, whisk together the egg yolks and caster sugar until pale, then whisk in the cornflour and plain flour. Pour the vanilla-infused milk onto the eggs, whisking continuously, then pour back into the pan.
Bring back to the boil, whisking continuously over a medium heat and cook for one minute. Pour the crème pâtissière into a bowl. Cover the surface with cling film to prevent a skin from forming and leave to cool. Transfer to the fridge to chill.
For the chocolate ganache icing, bring the cream to the boil in a small pan. Remove from the heat. Add the chocolate and stir until melted and shiny. Transfer to a bowl and leave to cool. Transfer the fridge to chill until the ganache has thickened to a spreadable consistency.
To assemble the religieuse, spoon the cold crème pâtissière into a piping bag fitted with a long thin nozzle (or alternatively you can use a jam syringe). Fill the choux buns with the crème pâtissière.
Dip the filled buns into the chocolate ganache to coat half-way up the sides. Sit the small buns on top of the larger buns.
For the collars, whip the cream in a mixing bowl until soft peaks form when the whisk is removed from the bowl. Spoon the cream into a piping bag fitted with a small star nozzle. Pipe lines of cream around the join where the small bun sits on top of the large bun to form a collar.