Whisk the yeast, warm water, and sugar. If using liquid milk, whisk it with yeast and sugar. Then let it proof for 5-10 minutes till it's foamy at the top.
Add the flour, powdered milk (if using), eggs, salt, vanilla, and the yeast mixture in a stand mixing bowl. You can mix manually if you don't own a stand mixer.
Mix on low speed for 2 minutes until the dough forms a ball.
Add the softened butter and mix again for 5-7 minutes until the dough is slightly sticky.
Let the Dough Proof
Lightly grease a bowl with oil and place the dough inside. Rub some oil on the dough so it dosen't dry or crusty.
Cover and let it rise for 1 hour 30 minutes.
Shape Donuts
Cut parchment paper into 12 x 12 sizes and place it on the working surface. Sprinkle some flour on it so that the dough balls don't stick.
Punch the dough to release trapped air bubbles.
Divide the dough into 10-12 portions and roll each portion to form a ball.
Place each ball on each parchment paper.
Cover with a cloth and let it proof again for 45 minutes.
Fry the Donuts
Add oil to a deep pot or pan over medium heat. You can use a thermometer to reach 375℉.
Add 2-3 donuts at a time and fry for 1 minute each side till golden brown. DO NOT OVER CROWD THE PAN.
Remove the donuts and place them on absorbent paper.
To make the Milky Filling
Combine the powdered, evaporated and condensed milk and mix well until smooth.
Transfer the mixture into a piping bag.
Using a knife, poke a hole in the side of each doughnut and fill it with the milk filling.
Serve as soon as they are filled because the milk may become harder inside the donuts.