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

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.

Next, add the softened butter and work it in with your hands until the dough is smooth.

Transfer the dough onto a flat, lightly floured surface and knead for two to three minutes.

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.

Finely dice strawberries and mix with 2 teaspoons of sugar, set aside.

In a small saucepan, add heavy cream, milk, and salt and cook stirring on medium heat until it just begins to simmer, turn off the heat.

In a separate large bowl, beat egg yolks and sugar until pale and combined.

Next, very slowly and stirring constantly, little by little pour the warm milk mixture into the egg yolk mixture.

In a small bowl, whisk together cornstarch and water until fully dissolved. Add the slurry to the custard mixture.

Transfer the custard to the saucepan and cook over medium-low heat, stirring constantly, until the custard is thickened and glossy about 3-5 minutes.

Remove from heat and add the butter and vanilla, stir the mixture until smooth.

After that, stir in the diced strawberries.

Transfer the strawberry custard to a bowl and cover it with plastic wrap, so it touches the surface of the custard, and refrigerate until cold and ready to use, for a few hours.

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.

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.

Place a large, deep pan with about 1 ½ - 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.

Remove them from the oil with a slotted spoon and place them on a wire rack to cool.

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.

Transfer the strawberry custard into a piping 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.