ASo a customer asked me to make a cake with sugar beer bottles on it. I've never done that before but I was up for the challenge. Working with isomalt was a little more work than I thought. I told her a price for the cake and then after doing all the work on it feel like I've totally under charged her. My question is do I go back and ask her for more money because the cake was more work than I thought? She is a very good customer of mine and don't want to lose her business.

ANo, i wouldn't do that. If I look at it from a customer's point of view, then it's not fair. If you go f.e. to a car work shop because your car has a problem, they'll either give you a quote and stick to it (more or less - if they are professional) or they call you to let you know that there is a bigger problem than expected and give you a new quote which you can then accept or decline. But imagine they tell you it'll cost you 200$ and when you go to pick your car up it's 400$

No definitely don't tell her it's more unless you're fine with her backing out on her end and then possibly losing her as a customer altogether. She agreed to the original price. It wouldn't be fair to tell her otherwise. You have to just chalk it up to one of those "now you know" lessons.

The customer is expecting to pay the price you quoted her.  It would be completely unprofessional to raise the price because you didn't charge enough.  You'll not only lose a customer, but the bad word of mouth will end up costing you even more.

Chalk it up to a lesson learned

AThe price you quote is the price the customer pays. You now know for the next time someone orders sugar bottles what to charge to compensate for your time.

that sucks -- an idea for you -- charge her the original quote but write up an invoice showing the full price you shoulda charged then a 'one time portfolio discount' of whatever the difference between that & the quote -- so the total due is the original quote --so she knows for next time and will know how much to tell others who might ask her -- can help avoid confusion -- enables you to get pictures for your portfolio and save face eliminate confusion -- and maybe inadvertently  generate a tip -- but maybe not-- 

but especially if she's a good client you want to keep her informed

AAbsolutely not.