A muffin is any quickbread (mixed by muffin mixing method & does NOT contain yeast) that is in the shape of a cupcake. A cupcake is any cake batter mixed with the straight method or creaming method made into the shape of a cup. Cupcakes (traditionally) have frosting on top, while muffins have no frosting, but muffins can have a glaze on top. Many people think that the difference is either frosting or no frosting, or that it's "in a paper liner" or not, but that is a common mistake. Either one can be made with or without a paper liner; it's just the baker's preference. A quickbread (or muffin) CANNOT be baked like a sheet cake or round layered cake because it won't bake properly. The same goes for a cupcake batter- a cake batter (or cupcake) cannot be baked in a loaf or bundt pan because it will not bake properly. But, since both can be baked into a smaller shape, everyone seems to want to declare that they're the same.
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