You can pour off the milk and spread what's left on your toast (salting is optional).

When the fat globules in whipping cream start to break, the fat leaks out and begins to coagulate in larger and larger clusters, which we call butter. There is no way to put those genies back in their bottles - no real way to return to a whipped-cream stage.

If you catch it just at the moment it begins to curdle, you can gently stir in a couple of tablespoons of additional cream and this may help. But you may still feel little bits of butter in the whipped cream, and, in most settings, that will be unacceptable.