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.
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.