If a muffin has long tunnels from top to bottom, what should you avoid doing next time?

The presence of long tunnels in a muffin, also referred to as "tunneling," is typically caused by over-mixing the batter. When the dry ingredients are combined with wet ingredients, the goal is to mix just until they are combined. Over-mixing can incorporate too much air and develop the gluten in the flour, resulting in an undesirable texture and the creation of tunnels as the batter bakes. By avoiding over-mixing, you allow the muffins to rise properly and maintain a more uniform crumb structure. This promotes even cooking throughout and achieves a desirable, tender muffin rather than one with large holes.

In contrast, adding more baking soda, decreasing the amount of butter, or adjusting the cooking time are not effective solutions for the tunneling issue. These factors might affect the muffin in different ways, such as altering its leavening, moisture, or doneness, but they do not address the core problem of how the batter is mixed.