It will eventually melt, it may take two to three days if you use something like Skor or Heath bar (commercially made “toffee”) but if you use your own caramelized sugar pulverized into bits it will melt sooner. The caramel pulls moisture from the buttercream and melts. You can approximate something like this using feuilletine, which is thin, almost impossibly thin shards of something along the lines of a crispy crepe. I am not sure you could make this stuff, I always buy it from a distributor. Or online at pastrychefcentral.com maybe. You could mix this into the buttercream and it will stay crispy for longer than the pulverized caramel; but for all that aggravation maybe just use your favorite toffee recipe (minus a chocolate coating) and see how long that lasts in the buttercream.