Can I substitute a German's sweet chocolate bar for chocolate chips in a rice crispy treat frosting recipe?
Yes, although you probably want to chop it up.
The German's brand is very sweet, but it is real chocolate and should work just fine.
I would suggest reducing any other sugar you have in the recipe by at least 1/4th to compensate for the extra sugar in the sweet chocolate bar. Also, give it a rough chop as you want pieces the size of chocolate chips