I've never had Portillos but chocolate cake from Bittersweet is like heaven. I had them do a chocolate cake with bittersweet chocolate mousse and chocolate ganache for my wedding cake and it was sublime.
We froze the part we were supposed to freeze and ate it a year later, and it was still heavenly.
I love Portillo's cake too. If you like that consistency and style, then Sweet Mandy B's is my second favorite for chocolate cake w/chocolate frosting ($15 for a small, which actually feeds a lot). It reminds me almost of a Hostess cupcake in flavor, but not texture. In my opinion, it's really, really good!
Sweet Mandy B's
1208 W. Webster Ave.