When it comes to racism, the causes are usually deeply rooted within the person's fiber and being. Many times it's a result of the experiences in their childhood, which consequently molds their personality. Unfortunately the older you are, the harder it to change your personality. It is likely your girlfriend's parents won't change their opinion, or at least for a while.
Thankfully, there is not much her parents can do to prevent a relationship between you two. Ultimately it is your girlfriend's and your choice on how you want to live your lives, and who you want to be with. But, I can understand why you would want an improved relationship with her parents, and I suggest you continue being a courteous, compassionate, and well mannered boyfriend, so that her parents will be more accepting of you. Continually make a good impression on her parents, and they may eventually change their minds.
Let it be known, if you screw up her parents will only use it as a reason for her not to see you. You'll be under constant scrutiny, and you have to make sure you make the right choices. (Ie, not getting into trouble, breaking rules, drugs, acting out, etc.) Just be mindful of your actions and continue being the great person and boyfriend you are. :)
I wish you and your girlfriend the best!
P.S. - Nice job at baking the cake, that's sure to get brownie points. ;)