Who wouldn’t choose the custard-filled doughnuts in a table full of treats? Sugar-coated with a neat stripe of homemade jam to finish, they are quintessentially British summer fête.
Make the dough. Gently warm the milk in a small pan over a low heat. Pour into a jug and beat in the egg.
Tip the flour into a mixing bowl and add the butter. Add the yeast to one side of the bowl and the salt and sugar to the other, then stir until combined. Slowly add the milk mixture, stirring by hand to form a dough. Knead in the bowl for about 4 minutes to bring together.
Turn out the dough onto a lightly floured work surface, and knead well for about 10 minutes, until the dough is smooth and elastic – it will still be quite sticky at this stage – then place in a clean bowl, cover with a damp tea towel and leave to rise for about 45 minutes, until doubled in size.
Meanwhile, make the jam. Put the strawberries in a small saucepan, add the jam sugar and slowly bring to the boil over a low heat. Crush the strawberries and sugar together using a potato masher, then when the sugar melts, increase the heat and boil for another 4 minutes, until the temperature on a sugar thermometer reaches 105°C/221°F. Remove from the heat and carefully pass through a sieve to remove the seeds. Leave to cool and set.
Meanwhile, make the crème mousseline. Heat the milk over a low heat in a heavy-based pan.
In a mixing bowl, whisk the egg yolks and sugar until combined. Sift in the plain flour and cornflour, then whisk until smooth. Whisk in the vanilla extract and salt.
Pour a quarter of the warm milk onto the egg mixture, whisking continuously, then pour the mixture back into the pan of warm milk. Return the pan to a medium heat and cook, whisking continuously, until the mixture begins to boil. Cook for 2–3 minutes to a thick custard consistency.
Remove from the heat and stir in the chilled, cubed butter until combined. Transfer to a heatproof bowl and leave to cool.
Meanwhile, beat the softened butter until pale and creamy. When the crème mousseline has cooled to room temperature, add the creamed butter and whisk until smooth.
Turn out the dough onto a very lightly floured work surface and knead it a few times to knock it back. Divide the dough into 6 equal portions, about 70g each, and form each portion into a 15cm-long finger shape, with rounded ends.
Place the dough fingers onto a floured baking tray and leave to prove, covered (use a proving bag if you have one), for about 30 minutes, until almost doubled in size.
Pour enough caster sugar into a wide, shallow bowl to coat the doughnuts and set aside.
Half-fill a deep-fat fryer or large, heavy-based pan with vegetable oil and place over a medium heat until it reaches 180°C/356°F. In batches of two at a time, carefully lower the doughnuts into the oil and fry for about 3 minutes on each side, until golden brown.
Remove the doughnuts with a slotted spoon and immediately roll in the sugar until coated – the hot fat helps the sugar to stick. Set aside and leave to cool while you fry the remainder and coat them in sugar. Leave to cool completely.
Using a serrated knife, make a deep cut lengthways down the middle of each cooled doughnut, without slicing fully in half.
Spoon the cooled crème mousseline into the large piping bag fitted with the medium closed star nozzle, twist the top to seal, and neatly pipe the custard along the middle of each doughnut.
Spoon the cooled jam into the piping bag fitted with a large writing nozzle, twist the top to seal and pipe a line of jam along the middle of the crème mousseline to finish.