Muffins, usually sweet cupcake-shaped breads, may require a double explanation in the UK. Typically, and until recently, muffin in the UK referred to what most call the English muffin: a flat round yeast bread that is split and toasted. The US type is very different, and is defined as a quick bread, since it doesn’t contain yeast. Many variants exist and both types are sold in the UK now. In fact the term may come from an old French word introduced into English, moufflet, in approximately 1000 CE. The term may have once been mufflins, and at least was so used by some Irish immigrants to the US in the early 20th century.
Muffins get their characteristic rise from baking powder, or sometimes baking soda, instead of yeast. No matter what type you are making, you can mess up a recipe with a few novice mistakes that are easily remedied. First, when you’re mixing muffin batter, even if you’re using a premade mix, you only want to mix until the ingredients are just combined. Overmixing causes tunnels and affects the fine crumb each muffin should have. Second, people err by overfilling muffin tins. Typically, you want to fill each tin to no more than 2/3 capacity.