This word is using a 'slang' way - not very common, but I think the speaker is referring to something like this--> It heats things up: like sandwiches with cheese in, so melting the cheese...😋

She is speaking quickly. She has had an idea. She is trying to communicate quickly, rather than precisely. She is not using standard English. She is speaking in an excited way. She wants to destroy something made of wax. When you heat wax, it melts. She is using both "hotty" and "melty" as adjectives. They both modify the word "things." It is almost as if she can't decide which to use so she just uses both. She is using them to mean "something that can make wax hot" and "something that can melt wax." "Melty" doesn't normally mean "something that melts something else." It normally is an adjective meaning "something that melts easily." "I use colby cheese in grilled-cheese sandwichses because it is more melty that cheddar cheese." There is a slang noun, usually spelled "hottie." It means an excitingly attractive person. She doesn't mean that here.

Hotty means hot and yes it's an adjective because it describes the things which will be used to melt them.