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

Why can't I believe it's not butter? - Quora

7 Reasons the Carnivore Diet is Unhealthy and What to Try Instead - Gene Food

What actually is I Can't Believe it's not Butter? - Quora

What actually is I Can't Believe it's not Butter? - Quora

I Can't Believe It's Not Butter! Original Spread, 45 oz - Smith's Food and Drug

What's the Difference Between Butter and Margarine?

What does it really mean by the saying that 'megadrought' is emerging in the Western US? - Quora

I Can't Believe It's Not Butter Fabio Commercial (1996)

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

I Can't Believe It's Not Butter! Class Action Alleges Product Contains Less Olive Oil Than Advertised - Top Class Actions

I Can't Believe It's Not Butter!®

The Best Brown Butter Chocolate Chip Cookies

50 Southern Phrases You Won't Hear Anywhere Else