Refrigerated ingredients like eggs and butter should be brought to room temperature so they will incorporate into the batter properly. This gives muffins a proper rise in the oven.
Leavening agents like baking soda and baking powder are less potent once opened for six months, so if your muffins don’t rise, it may be because your agents aren’t fresh.
To test baking powder, mix one tsp with ½ cup of warm water; if it bubbles, it’s good to go. For baking soda, mix it with some drops of vinegar or lemon juice to see if it fizzes.
Unlike measuring cups, using a scale to weigh ingredients will always give you spot-on measurements that are consistent. It also makes for an easier job with less cleanup.
To avoid texture issues, keep dry and wet ingredients separate until you combine to finish the batter, which will ensure the leavening agents don't activate prematurely.
Only mix your batter until all of the dry ingredient particles are incorporated, and no longer. Otherwise, air bubbles will be removed and result in dense, chewy, tough muffins.
Made of eggs, oil, and vinegar, mayo provides moisture, fat, and acidity that promote a better rise in muffins when mixed with baking soda, and help the proteins in the flour set.