Instructions

1. Line a 7" x 11" pan with parchment paper.
2. Put the butter and water in a LARGE, heavy pot and heat to melt the butter, then add the milk.
3. Over low heat, add the sugar and stir until it is completely dissolved. Bring the mixture to a boil and allow to simmer (keep boiling, but turn it down so it doesn't overflow the pot) for 10 minutes without stirring.
4. Add the condensed milk. Stir for at least 10 minutes while simmering, just be sure it doesn’t stick or burn. This is the critical part as you don’t want it to burn (the heavy pot is important for this).  TIP: the time to cook the mixture will vary upon many factors, but most importantly, how high your cooktop is set to. If you have it too high, the tablet mixture will stick and burn, too low and it will take ages to get to the right temperature. Keep it simmering until you see it start to turn a lovely golden color. 
5. Remove from heat and stir in the vanilla or whisky (I use whisky), and allow to cool for a minutes or two before beating vigorously until it thickens. Don’t be impatient or think you’ve failed as this could take a few minutes. .
6. When thickened, pour into the prepared pan. Allow to cool and set completely in a cool place, or the fridge.
7. Cut, then wrap in parchment paper for individual servings, or simply keep in an airtight tin in a cool place, or in the fridge.