Melt the butter in a large, heavy based pan over a low heat. Tip in the sugar and milk and keeping the heat low, allow the sugar to dissolve, stirring occasionally. When everything has melted together and there are absolutely no grains of sugar left, add the condensed milk and slowly bring to the boil, stirring constantly.
If you have a sugar thermometer, the setting point is 120c. Otherwise, keep a cup of cold water by the side of the hob. To test the tablet, drop a small piece into the water. If it forms into a soft ball, it's ready.
Now for the part where you'll need a bitof muscle - take off the heat, stir in the vanilla extract and beat vigorously for about ten minutes, or until the tablet is thicker and slightly grainy in texture. I've heard of some people using a mixer for this step which would save you from getting sore and tired arms!
Tip into the lined tin and smooth out as best as you can. I find a silicone spatula is best here.
Leave to set and cut into small squares.