The ideal time to leave a completed cake batter sitting before putting it in the oven can vary depending on the specific recipe and ingredients used. However, in general, it is best to bake the cake as soon as the batter is prepared and ready. Cake batters typically contain leavening agents like baking powder or baking soda, which start to activate as soon as they come into contact with moisture.
If you leave the batter sitting for too long, the leavening agents may lose their effectiveness, and the cake may not rise properly during baking. As a result, your cake may end up dense and flat instead of light and fluffy.
That said, some cake recipes may allow for a short resting period to allow the ingredients to hydrate and flavors to develop. If the recipe specifically mentions a resting time, it's important to follow those instructions. Typically, this resting period should not exceed 30 minutes.
If you encounter unexpected delays or need to prepare the batter in advance, it's usually better to refrigerate it and then bring it back to room temperature before baking. However, keep in mind that even with this method, the cake may not turn out as well as if it were baked immediately after preparing the batter. Always refer to the specific cake recipe for the best results.