I don't think it matters very much about salted or unsalted butter. I only make Soupdragon's flapjacks. Not keen on the type that fall apart like something you'd give to horses in a nosebag. Condensed milk for the win!
This is basically the Soupdragon recipe, copied and pasted and slightly elaborated.
SoupDragon’s condensed milk flapjacks
[Absolutely wonderful. Much better than ordinary flapjacks.]
250g porridge oats (the cheap type, not large ones)
150g Butter
2 generous tablespoons (75g) Golden Syrup
half a tin condensed milk
[I usually double up all ingredients so the problem of the half tin does not exist]
You'll also need an oven, a saucepan, some baking parchment and a large
baking tin or a couple of small ones.
Method:
Preheat the oven to 160 degrees C.
Line the baking tin with baking parchment, this doesn't have to be neat.
Melt the butter in a large saucepan over a medium heat.
Add the syrup and condensed milk. Keep heating and stirring until all is melted and mixed. Don't rush by turning the heat up.
Let the mixture boil for a couple of minutes, stirring to prevent it sticking to the pan.
Gradually add the oats, folding them in. All the oats should be coated, and the mixture quite dense, but still sticky. Don't add so many oats that the mixture becomes dry.
Pour the mixture into the tins and spread about so that it lines the tin to a depth of 2-3cm. Fill as many tins as necessary! Don't squash the mixture in, just spread it evenly.
Bake in the oven for 15mins. If using top and middle shelves swap half way through so they cook evenly.
You should take them out when they just start to go brown round the edges, don't leave longer than this. If they're still squidgy in the middle that's fine, they set on cooking. Cut into squares while still hot.