i tried to make yema.
i just boiled two cans of condensed milk for about an hour (the cans are washed and label was cut off and the can was submerged in a boiling pot of water) opened up the can, placed it in a bowl and mixed in two egg yolks while the condensed milk was still hot. you can form it in any way you want but the traditional is the pyramid shape and wrapped in yellow cellophane.
from what i remember you can just cook condensed milk in a pan and add some butter.
remember the condensed milk when cook is like caramel, only stickier or thicker and creamier.
Hope this helps!