Butter is typically used in one of three ways in a recipe: cold, softened or melted. Cold butter is typically cut in to a mixture of dry ingredients. Softened butter is used in recipes that require the butter to be creamed, such as cookie dough and frostings. Melted butter can be used in many recipes, and often it is stirred into a dry mixture along with other “wet” ingredients, such as eggs and milk. The vast majority of baking recipes call for butter to be cooled down after being melted and before being added to the rest of your ingredients, but very few recipes define how cool the butter needs to be before you can use it.
In just about all recipes, butter should be cooled down to just above room temperature before you use it. Depending on how much butter you’ve melted and how hot you made it while melting, this could take anywhere from 1 – 5 minutes. You don’t need to use a thermometer to check the temperature, but a basic guideline is that the liquid butter should be cool enough to handle easily and not so cold that it starts to resolidify. If a recipes needs the butter to be hot instead of room temperature, it should explicitly mention it.
Butter can be melted quickly in the microwave or slowly on the stovetop, and it can be made plain or cooked until it has become browned butter. When it is very hot, melted butter can actually melt the sugar in your recipe or even cook the eggs. In the case of the sugar, this can drastically change the texture of your finished product, and accidentally cooking the eggs can give your recipe flavors you didn’t expect (and definitely don’t want – especially where desserts are concerned).