Place sugar and full fat milk into large pan. Place over a gentle heat and stir occasionally until all the sugar is dissolved.
Once all sugar is dissolved add butter and allow to melt. Once butter is melted add condensed milk, mix in well. Increase the heat and stir continuously while mixture boils until it has reached Soft-Ball Stage - 120 degrees C / 240 degrees F (I use a sugar thermometer).
Allow mixture to settle and add a few drops of vanilla.
Beat mixture (I use a hand held beater) until it is almost setting in pot. Pour mixture into a well greased swiss roll tin and allow to set.
After 30 or so minutes I score the size of pieces I want, this makes it easier to finally cut the tablet when it is set.
Gas cookers / hobs melt the milk and sugar mixture far faster.
Use a sugar thermometer until you know what almost ready tablet looks like; this avoid disasters.
Be careful the boiling mixture is very hot.