Chocolate and blood orange custard doughnuts

1. Mix the yeast with 40ml warm water in a jug. Combine the flour, orange zest, sugar and salt in the bowl of a freestanding mixer fitted with a dough hook. Add the yeast mixture and the egg. Mix on a medium speed for 5 minutes, until it forms a springy dough (you can knead by hand for 10 minutes but it's quite sticky). Continue to mix while slowly adding ¼ of the butter at a time. Once it's all incorporated, raise the speed to high and mix for 5 minutes, until the dough is shiny and stretchy. Shape it into a ball, then return to the bowl. Cover with a clean, damp tea towel and leave in a warm place until doubled in size (about 1 hour).

2. Meanwhile, make the filling: in a medium bowl, mix the egg yolks, sugar and flour. Put the milk, orange juice and chocolate in a small pan over a low heat. Heat gently, stirring occasionally to melt the chocolate. Once melted and steaming, slowly pour this over the egg yolk mixture, stirring constantly (don't worry if it looks grainy). Rinse out the pan, then sieve the mixture back into the pan. Cook over a low-medium heat, stirring, for 3-5 minutes, until thick and smooth. Stir in the orange zest, then transfer to a clean bowl and cover the surface directly with baking parchment. Leave to cool, then chill. 

3. Once the dough has risen, knead it in the bowl to knock it back. Set aside for 3 minutes, then cut into 6 pieces. Roll into balls using your palms and space out on a floured baking tray; cover with a damp tea towel and leave in a warm place until doubled in size (1-2 hours). Meanwhile, whip the cream to soft peaks, then fold through the cold custard with a pinch of salt. Transfer to a piping bag; keep chilled. 

4. Once the doughnuts have proved, half-fill a medium, heavy-based pan with oil and bring to 160°C over a medium heat or until a cube of bread turns golden brown in 60 seconds (but no faster). Line 1 plate with kitchen paper and 1 with sugar. Once the oil is ready, use a palette knife to gently lower in a doughnut. Cook in batches of 2 or 3, giving plenty of space, for 2-3 minutes on each side until golden brown. Use a slotted spoon to lift onto the kitchen paper. Keep an eye on the oil temperature as you cook the remaining doughnuts. 

5. When cool enoough to handle (about 5 minutes), use a skewer to poke a hole in one side of each doughnut. Gently push in the tip of the piping bag and fill the doughnuts. They will expand a little and feel heavier. Roll in the sugar while warm and eat on the day of cooking.