I always thought you couldn't. In fact, I used to get stressed out if I had to wait even 5 minutes after mixing the batter. I'd get nightmares about baking a brick instead of a light cake if a few minutes were to pass without the batter getting into the cake tin and right into the oven. But two days back, when I baked the caramel muffins , I had some batter left over. Instead of throwing it away, I tried a little experiment. I stored the batter in the fridge for a whole day. 24 hours later, no major catastrophe happened. The cakes went into these cute little heart molds and rose perfectly. They were just a tad bit denser than my first batch, so there is some disadvantage to keeping the batter overnight. But even if I am not going to make a habit of storing cake batters, I now know it's not the end of the world. I can totally stop panicking when the doorbell rings right after I stir everything in and the cake has to wait an hour because a friend dropped by.