look as if butter wouldn't melt

You know very well how to do it, though you look as if butter wouldn't melt in your mouth!

I was sure that you would be on my side, though you do look as if butter wouldn't melt in your mouth.

The clouds are what no one can understand yet, I suppose," said Mr. Eildon, "but they don't always look as if butter wouldn't melt in their mouths, as they are doing to-day.