Add the water to a large pot then take the milk powder, and slowly incorporate it into the water, stirring gently with a slotted spoon.
Melt the butter until barely melted in the microwave or in a pot on the stovetop. Then take about a cup of the milk and heat it up until it is around 110F (43C) it doesn't have to be exact. Combine the two and either whisk it really well for 1 minute or put it in a blender and blend it for a minute.
Now slowly pour and add the buttered milk into the main pot of milk.
Heat the pot of milk slowly over medium-low heat until it reaches right around 90F (32 C). Stir gently so the milk heats up evenly.
Remove the pot from the burner.
Add 1/4 cup of filtered water or good tap water to a small bowl and then add the citric acid to it. Stir until dissolved. Add the citric acid to the pot of warm milk. Gently stir for about 30 seconds. You will begin to see curds form.
Then smash up the half tablet of rennet in a bowl with the back of a spoon, or use a mortar and pestle if you have it. Add that rennet powder to a bowl with 1/4 cup of water and mix until dissolved. Now pour it into the pot of milk. Again, gently stir for about 30 seconds.
Cover the pot, and allow it to sit for 30 minutes up to 2 hours. You really need to give the curds time to develop.
Once the curd has had time to develop, remove the lid and use a long knife, I always use my bread knife and cut the curd into 1/2 to 1-inch cubes.
Place the pot back on the burner and set the heat to medium-low. Instead of stirring the curd, rotate the pot which will move the curd around. Heat the curd until it reaches between 110 and 115 F (43 to 46).
Remove the pot and turn off the heat. Use a slotted spoon and scoop out the curd and place it in a cheesecloth-lined colander with a baking pan or sheet pan under it to catch the whey.
Fold up the corners of the cheesecloth and squeeze the curds, this will release the whey. Do this a few times until you get most of the whey out.
Pour the curds into a microwave-safe bowl. Place the pot of curd in the microwave and heat for 1 minute on high. This should bring the temp to right around 160 to 165F (71 to 73 C).
Dump any whey that seaped out of the curd. Smash the cheese and stretch it in the bowl, it will probably be too hot to handle at first. Then using clean hands or wear gloves, knead and stretch the cheese. You will wind up doing this for 3 to 5 minutes. You may need to put it back in the microwave as it cools down because the cheese will get harder as it cools.
Add in a bit of cheese salt, flaky sea salt, or crushed kosher salt. Then knead the cheese again so the salt gets mixed in.
Shape the cheese into a ball, it should look nice and smooth and a little shiny. There you go, you just made homemade mozzarella cheese from milk powder.
Place the cheese ball completely covered in a bowl of cool water for about 10 minutes. After that add some ice to the bowl to further chill the cheese for another 10 minutes, then wrap with plastic wrap and place in a sandwich back or Tupperware and store in the fridge.
Slice or shred and eat that mozzarella cheese made from milk powder.