In large mixing bowl add fish sauce, sugar, salt, potato starch and mix well. Add the finely ground pork and mix well until all the liquid has been absorbed into the pork. Take a teaspoon of the mixture and microwave for 20s and test the seasoning. Adjust to taste. Cover tightly and refrigerate overnight.
The next day, set a large steamer and bring to boil. You need to whip the mixture either with stand mixer with paddle attachment or food processor. Work in small batches and whip the mixture one more until it's glossy and smooth. It's very important to keep the mixture cold, so only work in small batches and keep remainder in the fridge. The texture should be very smooth--stop here if you're just making gio song.
If you want cha lua, take small portions and wrap in either banana leaf or in plastic wrap. Steam until the internal temperature of the cha hits 165. Please see video above.
You can substitute with other lean meats such as chicken. Fried cha lua is called cha chien.