Let Them Eat Cake....or Maybe Not.

My very first 100% homemade cake.  The actual cake was homemade, the strawberry filling and the cream cheese frosting was completely made from scratch.  I found this recipe for the cake on ChefTalk and it has numerous rave reviews.  I followed this recipe exactly as described on the link with one exception.  I used margarine in place of butter.  I put it in my oven at 350* for 25 minutes and when I took it out it still seemed as though it needed to bake a little more.  I placed it back in the oven for another 20 minutes and then let them cool as I made the strawberry filling.  That filling was very tasty and then I went on to making my cream cheese frosting.  Again, I did not use butter as the frosting recipe called for, I used margarine and I don't know if this is why it didn't turn out to my expectations, or what...just the only logical explanation I can give.  My frosting was runny.  And it didn't harden over time or when cooled in the refrigerator.  Hmmm...I will have to try actual butter next time and see if that was the true problem to my icing; and possibly my cake as well.  When the cake cooled I noticed that it hadn't risen very well and looked very dense.  I stuck my toothpick in and it came our perfectly clean so I just assumed at the time; because it is homemade, it doesn't rise as much as a boxed cake.  After dinner I decided it was time to cut the cake and I was quite nervous because I was afraid that it wasn't up to par. 
<---THIS is what I got. It was not fully cooked, even after cooking it double the time for what the website directions called for.  Needless to say, I was completely disappointed.  I had been looking forward to this cake for hours and wasted so much time and baking ingredients making it, such a let down.