Grease a suitable glass container thoroughly with vegetable oil.
Put milk and farmers cheese in a large saucepan with a thick base and heat over a medium heat, stirring with a wooden spoon, until the whey separates from the cheese in the milk. This produces a yellowish liquid and the farmers cheese takes on a slightly elastic, smooth consistency.
Hang a fine kitchen sieve over a large bowl, pour the milk mixture into the sieve and allow the cheese to drain briefly, stirring and turning carefully.
Return the cheese from the sieve to the saucepan, add butter and salt and turn and stir until the butter has melted.
Add egg and stir to form a homogeneous mixture.
Now heat the cheese mixture in a hot water bath for approx. 4 - 5 minutes, stirring constantly, until the farmers cheese has melted and a thick, smooth mixture is formed.
Add baking soda and continue to heat for a further 3 minutes, stirring constantly, until the cheese mixture becomes airy and increases in volume.
Pour the cheese mixture into the greased glass container, smooth it out and leave it to cool.
Close the glass container and leave the cheese to mature in the fridge for at least 24 hours, but preferably 3 days.