When the 20 minutes are up, check your cake with a toothpick.
If it comes out pretty much clean, it's done. If it comes out gooey and chocolatey, you stuck it in a chocolate chip and need to try again.
If it comes out gooey and doesn't look done and you'll need to judge this for yourself, then stick it back in for a few more minutes but keep and eye on it.
20 minutes has worked really well for me.
Place the cake pan on a rack to cool and continue cooking your other 2 pans in the same fashion.