A couple of things. Firstly, the cake is the smallest battle here. She just rocks up whenever and brings presents and junk? I'd deal with that first, establish boundaries with visiting and what she can bring. There's no need to bring stuff every time, it just shouts buying the childs love to me - never sat well.
The cake? Let your DH handle it if you absolutely want to make the cake. If not use the cake as the softening blow. How sweet she wants to make the cake, that's lovely and then while your husband tells her how nice it is he can throw in how unacceptable her surprise visits are.
Don't get involved until you have to. MIL that have a bit of a 'streak' about them love when their little boys wife fights with them. (experience lol)