What does America's proverb 'I can't believe it's not butter' mean

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If 'I can't believe it's not butter' is not butter, what is it ? Is it just better butter? How? - Quora

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I Can't Believe It's Not Butter!®

What is the native version of this Russian proverb: The same hammer that shatters glass forges steel? - Quora

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Can't Believe It's Not Butter Baking Sticks, 16 oz Paper Box 4 Sticks (Refrigerated)

What does the expression 'it's like butter wouldn't melt in his mouth' mean? - Quora