Buttermilk pancake batter leavened with baking soda should be mixed...

Buttermilk pancake batter leavened with baking soda should be mixed just before serving time to ensure optimal texture and rise. Baking soda is a chemical leavener that reacts with the acidity in buttermilk, producing carbon dioxide gas that causes the batter to rise and create a light, fluffy pancake. This reaction starts as soon as the baking soda comes into contact with the liquid ingredients.

If the batter is mixed too far in advance, the chemical reaction would begin and then diminish over time, leading to denser and less airy pancakes. Mixing the batter right before cooking ensures that the carbon dioxide bubbles are still active, resulting in pancakes that are light and fluffy. Additionally, this approach allows for the freshest flavor and texture, maximizing the quality of the final product.

In contrast, the other options could positively impact the pancake's quality: preparing the batter in advance, mixing it while the pan heats up, or after adding milk would all risk losing the leavening power of baking soda.