Hot fudge ice cream topping works great for filling. As long as it is a thick topping and not a runny one.

I think chocolate ganache would be your best choice. I just made a chocolate ganache over the weekend with milk chocolate and it was delicious!

Add another vote for ganache. It's basically as easy to make as just melting chocolate, but it stays smooth and creamy and tastes like heaven. If you don't mind an extra step you can whip it to make it a little fluffier.

Yes, I forgot to mention that part, whipped ganache is the best part.

I, too, would go with ganache.