In a large bowl, whisk together instant yeast, sugar & milk. Whisk until sugar dissolved & let the mixture sit for 5-8 minutes. It supposed to grow much bubbles as the sign that the yeast is active.

Crack the egg & add it into the yeast mixture. Then sift together the flour & salt into the bowl. Dump in the mashed potatoes & butter. Stir everything with a wooden spoon until well incorporated. (I use hand covered in plastic gloves to make sure there is no lumps).

Sit the dough for 5 minutes. (I use this spare time to wash all small containers I used). After that knead vigorously for 20 minutes, the dough should be elastic by now.

Round the dough & spray some oil all over it's surface. Put it back in the bowl & cover with some cling wrap to proof until double in size, around an hour. (At this step, I divide the dough into 4 parts then spray with some oil, put them each in a freezer safe clip bag and store in my freezer. The dough can be stored for 2 months).

Put the dough on a floured surface & punch it to release the air. Divide the dough into balls weighing around 25-50 grams. Put them on a floured tray & let then sit for 10 minutes.

Preheat the deep frier (I use traditional pan) & just before frying, use your finger to create a hole on the dough. Fry in a low heat & flip once after the bottom gets brown.