Heat the milk to 140°F over direct medium heat, stirring constantly.
After the milk has reached temperature, remove it from the stove and add the citric acid by sprinkling it over the surface of the milk and allowing it to dissolve for 30 seconds before stirring it in with a cheese spoon. Keep stirring the milk gently until it starts to separate.
Use a clean colander to drain off the whey and place the curds back into the warm pot.
To the curds add the soda, melted butter, salt, cheddar cheese powder, and finally the milk, stirring it all together gently but thoroughly with the cheese spoon. Turn the heat back on the pot, this time on low, and begin to cook the mixture together, stirring and cooking until all of the lumps are dissolved.
Put the cheese into a bowl and cover with a lid. Place the bowl in the fridge to set. When it has chilled thoroughly, it will be a soft cheese that melts into a creamy sauce.