Melt the butter in a stock pot, add the leeks and onions and fry until golden brown (about 10 minutes).
Add in the yams then cover with plenty of water and season to taste with salt and freshly-ground black pepper.
Bring to a simmer, cover and cook for about 30 minutes, or until the yam pieces are tender.
When the soup is almost ready, heat the groundnut oil in a pan.
Cut the bread into large cubes and fry in the oil until golden brown all over.
Remove the bread with a slotted spoon and set aside on a paper towel to drain.
When the yam mixture has cooked, take off the heat and allow to cool slightly then puree in a blender until smooth.
Return the puree to a pan and stir in the condensed milk.
Allow to heat through then ladle into warmed soup bowls and serve topped with the fried bread croutons.