The trick to making thin, tender peanut brittle is to keep the baking sheet(s) you use
warm. I recommend heating them in a conventional oven at around 200
o F (about 95
o C) before you spread the peanut brittle mix. This should allow you to spread the mix 1cm to 2cm thick without it setting up.
You should also keep a close eye on this recipe. Because of the variations in microwave heating power we've had a few reports over the years of people who've applied the power a little too enthusiastically and had this recipeexplode over the inside of their microwave. So when cooking this up, err on the side of caution!