When baking quick breads and cakes, can I premix wet ingredients the night before? When baking quick breads and cakes, I've premixed dry ingredients the night before to save time. Can I premix wet ingredients as well?

I understand that I can't premix the full batter when using leavening agents -- I am curious if I can save some time the day of while preparing the batter by premixing both the wet and dry ingredients separately beforehand.

The following is an example of a recipe for which I was thinking of doing this (wet ingredients would be the egg and buttermilk mixture):


  Ingredients
  
  1 1/4 cups buttermilk, milk, or yogurt (or 1 1/4 cups milk plus 1 tablespoon white vinegar; see Step 2), plus more as needed
  
  2 tablespoons butter or extra virgin olive oil
  
  1 1/2 cups medium-grind cornmeal
  
  1/2 cup all-purpose flour
  
  1 1/2 teaspoons baking powder
  
  1 teaspoon salt
  
  1 tablespoon sugar, plus more if you like sweet corn bread
  
  1 egg
  
  Instructions
  
  
  Preheat the oven to 375°F.
  If you’re using buttermilk, milk, or yogurt, ignore this step. If not, make the soured milk: Warm the milk gently—1 minute in the microwave is sufficient, just enough to take the chill off—and add the vinegar. Let it rest while you prepare the other ingredients.
  Put the butter in a medium ovenproof skillet or an 8-inch square baking pan over medium heat; heat until good and hot, about 2 minutes, then turn off the heat. Combine the dry ingredients in a bowl. Mix the egg into the buttermilk. Stir the liquid into the dry ingredients (just enough to combine); if it seems too dry, add another tablespoon or two of buttermilk. Pour the batter into the prepared skillet or pan, smooth out the top if necessary, and put in the oven.
  Bake about 30 minutes, until the top is lightly browned and the sides have pulled away from the pan; a toothpick inserted into the center will come out clean. Serve hot or warm.
  
  
  Source: Good Old Fashioned Corn Bread by Mark Bittman
Sure.

With quick breads you just don't want to activate your leavened prematurely. That happens when the wet and dry mix so you're fine.

Two things to watch out for:

The eggs and buttermilk are going to be more prone to spoiling after they are removed from their containers. Additionally, the batter could pick up funny flavors from the fridge. Storing in an appropriate container in the fridge and using it the next day would avoid any issues.

The wet ingredients, including the eggs, are emulsified when they are mixed. The oil dissolves into the buttermilk with the help of the egg yolks. This emulsification is important for the texture of quick breads. The emulsion is not indestructible. If you find your oil has separated in the morning you may need to beat it again to reemulsify before combining with the dry.

Note: your title says "and cakes". Cakes are assembled with different methods such as creaming or beating egg whites. These could not be done in advance without destroying the texture of the cake.