Slit the vanilla pod open lengthways and scrape out the seeds. Put both pod and seeds into a heavy-based saucepan with the milk and bring slowly to the boil, to infuse the vanilla.
Meanwhile put the egg yolks and the 125g sugar in a bowl and mix together for a few secs, then add in the flour and mix again.
Pour the just-boiling milk over the yolk mixture, whisking constantly to prevent curdling, then return the mixture to the saucepan. Cook over a medium heat, whisking constantly, for about 10 mins until very thick
Pass through a fine sieve, discarding the vanilla, and place a sheet of cling film on the surface of the custard to prevent a skin forming. Leave to cool, then chill in the fridge.
Whip the cream and the 2 tbsp sugar together until thick but not overwhipped, fold into the chilled custard. You can now scoop the the custard into a piping bag and use it as the filling in Justin's doughnut recipe.
Try different flavours...
Brown sugar: Replace the caster sugar with half soft dark brown sugar and half light brown sugar. You can add chopped stem ginger to the finished custard, or some hazelnut praline. Finish with half the quantity of cream.
Chocolate: Whisk 150g dark (70%) chocolate into the milk. Finish with half the cream.
Coffee: Add 4 tbsp of freshly ground strong coffee to the milk.
Malt & vanilla: Mix 2 tbsp of powdered malt into the sugar, and 2 tbsp of liquid malt into the milk.
Saffron: Add a good pinch of saffron to the milk. Finish with half the quantity of cream.
Violet custard: Add 3 tsp of violet extract and 3 tbsp of violet liqueur to the finished custard. Sprinkle sugared violets and crushed Parma Violet sweets over the top of the filled doughnuts.