Is it possible to freeze leftover Guinness? Seems a waste to have to throw away half a can after one measure for the Chocolate Guinness Cake. I know it wouldnt be suitable for "drinking" but maybe for the next cake? Thank you!
It is possible to freeze leftover beer and wine, but these are only suitable for use in cooking. It is useful when you only need a small amount of wine, such as when making a risotto, to be able to scoop some from a container in the freezer (the alcohol content in beer and wine means that they do not freeze solid but remain slushy).
For beers, such as Guinness, you will find that the liquid looses its fizz on freezing. So when using up the frozen Guinness you will need to consider the individual recipe. Unfortunately for the Chocolate Guinness Cake the carbonation can help to give the cake a bit of a lift, so it may not be a good idea to use the Guinness for baking. However, you could use it successfully in stews and casseroles, such as Nigella's Beer Braised Beef Casserole (from Kitchen).
You can thaw the frozen beer before using it, though for stews it is sometimes easier just to add the frozen beer directly to the pan and let it melt quickly over a medium heat.