White House Chef -- President Barack Obama's Go-To Meal ... A Good Burger

When the leader of the free world is jonesing for comfort food ... he ignores his wife's advice and reaches for a juicy hunk of grilled cow flesh ... so says the man who knows.

White House chef Sam Kass spilled this state secret to our TMZ photog. Fact is ... it doesn't necessarily square with Michelle Obama's campaign for healthy eating.

But Michelle has clearly had an influence on her food guru. Watch the video.