Shake Shack's Seattle secret menu is pretty trimmed-down (goodbye, peanut-butter bacon burger), but there's one item you should definitely know about.

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For all the mystique surrounding Shake Shack’s secret menu, it isn’t so hush-hush when you order in Seattle.

That’s because at its Denny Triangle location, the burger chain uses cashless kiosks, where customers use the touch screen to customize their orders. This means that most of Shack Shake’s off-menu items, such as the grilled-cheese sandwich, are available at a push of a button, just like any other food item. Just check off which fixings and ingredients you want on your burger or hot dog.

The Seattle location is only one out of nine Shake Shack outlets experimenting with this cashless kiosk system (out of 124 Shake Shack branches worldwide), which saves you the trouble of trying to decode the off-menu vernacular.

But I’m afraid I am the bearer of one piece of bad news.

Try as I might, I couldn’t find the button on the automated ordering system for the peanut-butter bacon burger, arguably Shake Shack’s most famous secret item.

A staffer — who looked as if she was about to inform me of a family death — said she was “very sorry” to say that Shake Shack got rid of that secret burger in September due to concerns over peanut allergies.

She paused, waiting for my reaction to see if I needed further consoling.

Hopefully, Shake Shack Nation can get through this mourning period and soldier on.

The good news is that there is one secret burger, not listed on any menu — and that is its Quad Shack. It’s a four-patty version of Shake Shack’s signature burger, with lettuce, tomato and tangy sauce.

The Quad ($13.99) can’t be ordered at the self-service kiosk (or by app, newly launched this week). This special order must be placed with a manager or staff member. That might sound like a marketing ploy, but a couple of staffers explained that the Quad is not part of its core menu, and ordering it slows down its service and doesn’t benefit the restaurant. The Quad has become such a fixture among the Shack Shake die-hards that they still make it upon request, a couple of staffers told me.

The Quad Shack is the equivalent of In-N-Out’s secret 4×4 burger. Only better. Much, much better.

Shake Shack’s four-patty burger is richer than an equivalent 16-ounce burger because the chain uses a ground-beef mixture with a high fat content — at least 20 percent fat, I’m guessing. And the creamy, melted American cheese is so thick that it doubles as an adhesive to keep the stack of patties in place.

If this is too rich, you can order the Triple ($11.19) Or request just the plain Quad Burger ($12.79).

There’s also a vegetarian item that’s part of the secret menu: the Garden dog ($2.69), which turned out to be awful, just a hodgepodge of hot-dog condiments without the meat. A better option is to order the ‘Shroom ($7.29) from the regular menu, a portobello mushroom cap cut in half and stuffed with cheddar and Muenster cheese and coated in egg, flour and panko crumbs. The result is an umami bomb, a crunchy fried shell to contrast with the gooey center that tastes like a runny yolk.