Butter can be used in many different forms in a recipe. Cold butter is usually cut into flour for pastry or biscuits. Softened butter is usually creamed with sugar for cookies and cakes. Melted butter typically used for cakes and quick-mixing recipes like muffins and quick breads. Recipes call for butter in different ways. When you’re dealing with solid butter, measuring is easy, but I often get asked “how are you supposed to measure melted butter in a recipe?” Some recipes call for “butter, melted” and others call for “melted butter,” and it can be a bit confusing at times.
The short answer is that you measure the butter before melting it, then you melt it and add it to your recipe. This is definitely the most common way of measuring melted butter and it is most likely the way that the person writing your recipe intended for you to do it. A very small amount of weight might be lost when you melt the butter (especially if you melt it at a pretty high temperature) because a little water will evaporate as the butter melts, but this should not have a measurable impact on the finished recipe. The only times when you want to melt the butter first and then measure it out are when your recipe calls for butter that is primarily used in a liquid form, such as browned butter, clarified butter or ghee.