Melt butter in milk, and add sugar.

Bring to boil and boil 10 mins. You shouldn't be able to feel any sugar crystals when stirring. It'll boil up and you need to keep beating it down so you can't leave it.

Add condensed milk, bring to boil and boil for 30 + mins It'll darken. Put a bit on a side plate and put some cold water on it. Once cooled, if you slide your finger over it, it should hold its shape and so it should set.

Remove from heat, add vanilla (about a teaspoon) and beat for 10 mins. You're looking for it to thicken and cool down in pot. If you pour to quickly into baking trays (greased) then it crystallises and goes graining. If you like it softer then don't let it boil for so long.