01 -
In a large bowl, whisk the warm milk with the room-temperature egg. Once combined, add the instant yeast and whisk fully until dissolved.
02 -
Add sugar, flour, and salt to the yeast mixture. Mix with a spatula until combined, then knead by hand until all flour is incorporated. Add the softened butter and knead until the dough is smooth. Transfer to a floured surface and knead for 2-3 minutes. Place dough in a lightly oiled bowl, cover with plastic wrap, and let rise in a warm place until doubled or tripled in size.
03 -
Heat milk and lemon peel in a medium, heavy-bottomed saucepan over medium heat until steaming but not boiling. Let stand 10 minutes to infuse. Whisk the eggs and sugar until pale and combined, then sift and add cornstarch and salt. Gradually whisk in the hot milk while whisking continuously. Return mixture to the saucepan and cook over medium heat, whisking until thickened (do not boil). Remove from heat, stir in butter, and cool with plastic wrap touching the surface. Refrigerate until ready to use.
04 -
Punch the risen dough and transfer to a floured surface. Fold a few times and divide into 8 equal portions. Shape into balls and place on a parchment-lined baking sheet, spaced apart. Cover with plastic wrap and let rest for 30-40 minutes.
05 -
Heat 1 1/2–2 inches of oil in a large deep pan to 350°F. Fry a few donuts at a time for 1-2 minutes per side until golden brown. Remove with a slotted spoon and cool on a wire rack.
06 -
Once cooled, poke a hole in each donut and roll in sugar. Transfer lemon curd to a piping bag, cut a small tip, and fill each donut. Ensure a small amount of custard is visible on top.