Yeast mixture:

Cup of Yum
1. In a large bowl, whisk the warm milk with the room-temperature egg, once combined add the instant yeast. Whisk fully until dissolved.

Donut dough:

1. Then, add sugar, flour, and salt to the yeast mixture. Use a spatula to mix until combined and after that knead with your hands until all flour has been incorporated.
2. Next, add the softened butter and work it in with your hands until the dough is smooth.
3. Transfer the dough onto a flat, lightly floured surface and knead for two to three minutes.
4. After that, transfer the sticky dough ball into a large lightly oiled bowl and cover it with plastic wrap. Place it in a warm place and let it rise for a few hours until it is about two to three times its starting size.

Homemade lemon curd:

1. Place a medium, heavy bottom saucepan, over medium heat, and add the milk and lemon peel.
2. When the milk starts to steam, please remove it from the heat, not allowing it to boil. Let stand for 10 minutes for the lemon peel to infuse the milk.
3. Meanwhile, in a large bowl, whisk the eggs and sugar until pale and combined.
4. Sift cornstarch and salt together, and add them to the egg mixture. Whisk on medium-high speed for about 5 minutes or until the mixture becomes pale.
5. Remove the lemon peel from the milk and discard it. Make sure the milk is still hot, do not let it cool.
6. While whisking continuously, pour the milk in a very slow stream into the egg mixture to tamper it. Gradually add all the hot milk while whisking.
7. Transfer the mixture to the saucepan over medium heat, and cook it whisking continuously for about 5 minutes. The mixture will thicken, but do not allow it to come to a boil.
8. Once the mixture has thickened, please remove it from the heat, and let it cool down for a few minutes. After that stir in the butter until fully incorporated.
9. Transfer the mixture to a glass bowl, and place a piece of plastic wrap in direct contact with the surface of the custard, to prevent a custard skin from forming.
10. Allow it to cool to room temperature for 15-20 minutes and then refrigerate until ready to use.

Shape the donuts:

1. In the meantime, punch the dough and then transfer it to a lightly floured surface. Fold it a few times and divide it into eight equal pieces.
2. Shape each piece into a ball and place it on a baking sheet covered in parchment paper. Repeat with the remaining pieces. Make sure they are not too close to each other on the baking sheet. Cover them with plastic wrap and let them rest for 30 to 40 minutes.

Deep frying donuts:

1. Place a large, deep pan with about 1 1/2 - 2 inches of oil over low-medium heat. Bring the oil to 350 degrees F. Add a few donuts at a time and fry them for 1-2 minutes on one side, or until golden brown. Flip and repeat.
2. Remove them from the oil with a slotted spoon and place them on a wire rack to cool.

Fill:

1. After the donuts have cooled completely, poke a hole, using a long rounded object, like a chopstick. Place a bowl with about one cup of sugar next to you. Then roll them in sugar until covered on both sides.
2. Transfer the lemon custard into a pipping bag, cut a small hole at the end, and fill the donuts with custard until you have just a little ball of custard on the surface.