In a frying pan or pot on medium heat, melt the butter.
Mix the egg yolks, condensed milk, and vanilla with the butter until well blended. Then add the mixture to the pan.
Add the peanuts and bring to a boil, then reduce to low heat to prevent the mixture from burning.
Using a metal spatula, stir the milk as if you are scraping the bottom of the pan. Stir constantly.
This must be done continuously to avoid burning the milk and to prevent it from sticking to the bottom of the pan.
The mixture is ready when it becomes very thick and ready to form as a ball. Usually around 20 mins or so. Let it cool.
When cool, form the mixture into balls about 2 to 3 cm in diameter.
Place the Yema balls on a wide plate with white sugar then roll; once coated, pick up each ball with a toothpick so all the sugar stays on.
Then wrap the balls in cellophane and twist both ends to secure, or you can get really creative with the presentation!