I am not sure what caused this cake to undercook on the bottom only.  I used a white cake scratch recipe from Cook's Illustrated.  It is one that uses reverse creaming.  I really enjoy the flavor. 

The toothpick came out clean, but that was misleading...when I cut into the cake after cooling, the bottom was undercooked.  I checked and my oven temp is okay.  I used the middle oven rack.

what the heck?

I'll try to add a pic but I'm not good at that.

you did great with the picture

and i'd guess that it was some kind of mixing snafu

are you sure your oven is cycling properly and holding the temperature constant?

looks like it was not hot enough then got too hot (cracking on top)

maybe

The crack is actually a cut ontop.  I cut it in half to check on it, since I wasn't going to layer it.  I'm glad I did, cause I would not want to serve something like that.    Thanks for your help anyway.

http://*********.com/learn/baked-goods/cakes/problems-and-solutions/butter-cakes

Is this a recipe that you have had success with before?

Your cake doesn't actually look underbaked.  Rather it looks like it has a dense layer on the bottom which could be due to a number of causes.

The above link is a troubleshooting page which suggests the following:

HEAVY LAYER ON BOTTOM
Not enough mixing
Too much liquid
Too many eggs

THICK / HEAVY LAYER
The butter, sugar and eggs were not beaten together long enough.
The flour was beaten at too high a speed.
Too much flour was added to the creamed mixture.
The oven temperature was not hot / or too hot enough.
Excessive baking time. Check cake at the beginning of the time range for doneness or 10 minutes before the stated time.
 

HTH

Apparently the site I linked is not permitted.  If you want, PM me and I'll send you the link.