Check the instructions on the yeast packet for activation.

In a mixer bowl, combine flours, sugar, and yeast.

Scatter butter cubes over the flour, then add eggs, lukewarm milk, vanilla, and salt.

Use a dough hook on low speed to knead for 10 minutes until smooth and elastic.

Tip the dough onto a clean surface and shape into a ball with minimal kneading.

Place the dough in a greased bowl, cover with plastic wrap, and let prove for 2-3 hours until tripled in size.

Knock air out of the risen dough, knead briefly, and roll out to a ½ inch (1.5cm) thick rectangle.

Cut out 3.5 inch doughnuts using a cookie cutter and place on parchment paper. Cover and prove for 1-1.5 hours.

Prepare the pastry cream while doughnuts are proving.

In a saucepan, combine egg yolks, milk, cornstarch, sugar, vanilla, and lemon zest. Cook over medium heat until thickened, whisking constantly.

Heat oil in a deep pan to 170°C/337°F.

Drop doughnuts into oil, remove parchment, and fry in batches for 2 minutes per side.

Drain on kitchen paper for about 2 minutes, then roll in sugar and let cool before filling.

To fill, make a small incision in the side of each doughnut, then pipe in pastry cream.