It's a question I get asked occasionally, and the quick answer is no, you can't make muffin batter ahead of time and bake them whenever you want. The reason for this has to do with leavening. Baking soda and/or baking powder need to be combined with a liquid to begin their chemical reaction that leads to rising. If you let the muffin batter sit for too long before baking, the muffins will likely not rise as they should.