How do you make chocolate stick to brittle?

I having been making a cashew brittle over the holidays and decided to add a chocolate base. I melted milk and dark chocolate together and spread it out on the flat side of the brittle after it had cooled. Once the chocolate hardened and I went to break up the brittle, some pieces of the chocolate stuck, and some didnt. I dont know what made it stick on some and not on other pieces, so im just wondering if there’s anything that I need to do to make more of it stick, or if thats just how it is. I slowly micorwave melted the chocolate and I had it outside cooling (so it may have still been cool, but had been inside for a little while)

Also, what can you use in the chocolate, so that its doesnt melt so quickly when touched (similiar to candy bars and such), i’ve heard paraffin mentioned before.