Roasted pistachio paste is a specialist ingredient, but worth taking the time to find. There are plenty of online stockists selling high-quality pistachio paste.

1. Split the vanilla pod open lengthways and scrape out the seeds. Put the seeds, pod and milk into a heavy-based saucepan and bring slowly to the boil, to infuse the vanilla.

2. Meanwhile, place the egg yolks and sugar in a bowl and mix together for a few seconds, then sift in the four and mix together.

3. Pour the boiling milk over the yolk mixture, whisking constantly to prevent
curdling, then return the mixture to the pan and cook over a medium heat, whisking constantly for about three minutes, until nice and thick.

4. Pass through a fine sieve (strainer), then place a sheet of baking paper or a scattering of caster sugar on the surface of the custard to prevent a skin forming (don’t forget to keep your vanilla pod – wash and dry it to use again).

5. Leave the custard to cool, then refrigerate until cold.

6. Remove the cooled custard from the refrigerator and give it a good mix to break up any lumps.

7. To finish, whip the cream and sugar together until thick, stir in the roasted pistachio paste until fully combined. Then fold in the chilled custard until well combined.

8. Transfer to a piping bag and you’re ready to fill your doughnuts

9. When you have piped in the filing, scatter over a few crushed pistachios to decorate.