The recipe, Freckled Mocha calls for 3/4 cup milk but I don't have any. I do have Buttermilk and Heavy Whipping Cream.
I just don't want to have to run to the store.
I would use the cream before the buttermilk. Buttermilk has a tang that might not be good in your particular recipe. I have used heavy cream and some water in place of milk, and it was fine.
Buttermilk is more acidic than milk. It will mess up your levening. Baked goods with buttermilk need the leavening adjusted to compensate for the buttermilk. As JoAnnB said, cream mixed with water is a better option. Your cake is probably already done, but I thought I'd throw in my two cents for others who might be reading.
I have used buttermilk in my chocolate cakes with great success, and sour cream, but never cream