Melting butter in the microwave, although great for other purposes that are strictly based on taste, isn't the best for lining your cake pans with. When you heat butter in the microwave, it causes it to separate. When you go to dip in a pastry brush or paper towel for coating you pans, you tend to just get the moisture or water from the stick, instead of the fat (which is what your cake needs for easy removal).
You can soften it in the microwave, just make sure to use 30-50% power instead of full strength. Use a pastry brush to spread it on and you're in business! Using a cold stick from the refrigerator works, but doesn't often leave behind enough butter to truly give that outer layer of cake the final texture it needs.
Give it a try next time you bake and see if it makes a difference for you!
• Related: 4 Ways To Reuse Your Butter Wrappers!