When baking you will find that some recipes call for softened butter to be used in their ingredients. Please be aware that softened butter and melted butter are entirely different things and should not be confused as one of the same.
Softened butter, is butter that has been taken directly from the fridge and left to rise to room temperature on its own, this type of ingredient can be used in buttercream as the icing sugar absorbs straight into the butter and milk giving you a creamy filling for your cakes.
Melted butter, on the other hand, is either melted in the microwave or a pan and usually mixed with other ingredients to give a coating or used as an adhesive to bind mixtures together.
As melted butter is warm and liquid, it could not be mixed with icing sugar as you’d end up with a sticky mess, the same should be remembered if you were to mix melted butter into batter. Because the butter is hot or warm, it could potentially start to cook your eggs when they are being incorporated into the mixture and may alter the final consistency of your baked goods. Therefore, please remember to use the correct method.