For the doughnuts, beat the milk, egg, sunflower oil and lemon juice together in a large jug. In a large mixing bowl, combine the flour, sugar and baking powder. Stir the wet ingredients into the flour mixture until combined as a smooth batter
Brush a mini doughnut pan with sunflower oil, then spoon Spoon a little of the batter into the holes, to just under half full – you may need more than one pan or to bake the doughnuts in batches. Bake in the preheated oven for 4 – 5 minutes
Fill a shallow dish with caster sugar ready for coating the doughnuts. Remove them from the pan whilst warm and toss the doughnuts in the sugar thoroughly to coat. Set aside
For the jam, cut the rhubarb into 1cm pieces and place in a pan with the sugar and water, bring to the boil and simmer gently for 10 minutes with the lid on
Remove the lid and stir, then remove from the heat when it reaches a jam consistency – you could serve this slightly warm as a fruit compote, or allow to cool and set
For the custard, add the milk to a saucepan, then scrape in the seeds from the vanilla pod and bring to the boil
In a bowl, whisk the egg yolks, sugar and cornflour together, then add the hot milk to the egg mixture and whisk together. Return the mixture to the saucepan and bring slowly to the boil, whisking continuously, until thickened
To serve, divide the custard between four espresso cups and add a spoonful of jam on the top. Serve the mini doughnuts on the side, ready to be dunked!