I based this recipe on this recipe by Laura’s Bakery 

1. 24 hours in advance: put the condensed milk in a bowl. Stir in 10 drops of Intoleran lactase drops. Cover and put in the fridge. Leave for 24 hours.
2. After 24 hours most of the lactose is digested and this product is safe to use for most people with a lactose intolerance.
3. On the day you are going to make the fudge: place all ingredients except the sea salt in a saucepan.
4. Heat over low heat until the sugar dissolves and the butter melts (3-4 min).
5. Then turn the heat to medium high and keep stirring the mixture continuously.
6. Keep stirring for about 15-20 minutes. Scrape the bottom well, so that nothing burns. The fudge is done when it is thick and gets harder to stir. It shouldn’t be runny anymore. This took me 20 minutes.
7. The cooking time can differ if you use a different type of heat or if you have a thicker or thinner pan. The cooking time will be between 10-20 minutes. 
8. Remove the pan from the heat and let stand for a few minutes. Add the sea salt if you like.
9. Stir the mixture for another 5 minutes (note: do not reheat, you do this with the pan off the heat).
10. If the fudge goes back very slowly when you draw a line through the mixture, it is thick enough.
11. You can now taste the fudge to see if you find it salty enough. Otherwise you can add some extra sea salt flakes.
12. Note: the fudge is hot. So don’t put your finger in it, but put a little on a spoon. Then blow very well until the fudge has cooled down before putting it in your mouth.
13. Take an oven dish and line it with baking paper. Spoon in the fudge and spread it over the bottom with a spatula.
14. Cover and let sit at room temperature for a few hours to allow the fudge to cool.
15. Cut into cubes and serve!