A messed-up mac and cheese order last summer turned into a happy accident for California Pizza Kitchen.

The Costa Mesa chain is now offering its mac and cheese fans “Mac It!” — any of their pizzas built with a mac and cheese base instead of crust for takeout and delivery.

Two pizzas have made it onto the CPK Favorites page of its website: The Original BBQ Chicken Mac ‘N’ Cheese and Burnt Ends Mac ‘N’ Cheese.

These creations take CPK back to its beginning and into its future, according to Chief Executive Officer Jeff Warne.

“It’s really of the moment. Mac and cheese is incredibly popular. And when you put our pizza flavors on it, it’s next level.”

CPK marked its 40th anniversary recently. On that day in 1985, founders Rick Rosenfield and Larry Flax opened the first restaurant at 207 S. Beverly Drive in Beverly Hills.

Warne met with them for a toast in the afternoon March 27 and then went on to a full-blown birthday party, complete with black balloons, at the CPK in Manhattan Beach.

The Beverly Hills restaurant has the smallest footprint in the chain, while the Manhattan Beach store is one of the largest among almost 200 worldwide.

Throughout 40 years, CPK has maintained its core values, according to Warne. The menu, however, is in constant flux, and there have been several leadership changes. Rosenfield and Flax were out and then back in and are now retired.

“What’s changed? It may be easier to ask what hasn’t changed,” Warne said.

“I really got an education today, spending a lot of time with Rick and Larry. I’ve interacted with them, but I really, really heard their philosophy from them. At the start, the hallmark of the brand was food innovation. That was the one thing they really wanted to say. This is what we’re going to be about. And I think we’ve carried that on for 40 years,” Warne said at the party. “I think these guys were true visionaries when they launched this brand.”

The pair were former federal prosecutors who got the urge to get into the restaurant business, believing they could do better than “thin, rubbery cheese pizza spinning under a heat lamp,” as they wrote in their 1995 “California Pizza Kitchen Cookbook.”

They actually came along as Wolfgang Puck was making his mark with gourmet pizza at Spago on the Sunset Strip and Alice Waters was publishing a pizza cookbook.

“They saw elements of fine dining where there was just one restaurant, and they said, if we can do that quality food with a unique twist on an item, make it a little more approachable,” Warne said. “They thought they had a winner and they certainly did.”

With a unique yet affordable menu that included items like duck pizza, CPK grew rapidly. It was so much of its time that it became part of O.J. Simpson’s murder trial in 1995 when Flax invited the jury to dine at one of its restaurants and most members accepted, as celebrity crime writer Dominick Dunne wrote in Vanity Fan. The day after, many of the jurors wore CPK T-shirts into the courtroom.

Most of the original menu is gone, but barbecue chicken pizza remains a bestseller and available frozen at grocery stores. CPK made a licensing deal with Kraft Foods in 1997. Nestlé acquired it in 2010.

“That one is our Big Mac,” Flax told a Denver Post reporter in 2000.

Paul Pszybylski, vice president of culinary innovation, was tasked with doing something special on the menu for the 40th anniversary. “We decided to pull favorites from each of the four decades that we’ve been in business,” he said.

Restaurant guests picked a Waldorf chicken salad, the office staff picked tortilla spring rolls, Pszybylski chose chicken Milanese, and the founders settled on rosemary, potato and grilled chicken pizza after Pszybylski called them individually.

“Larry said, here’s the thing. I never liked rosemary, but it’s a good pizza. So I’ll stand behind it. I called Rick, and said, ‘I don’t want to sway you but I’ve got one and I want to talk to you about it, so hear me out. What do you think of bringing back the rosemary potato pizza?’ He said, ‘That’s the one thing I wanted you to bring back.’ They both have their own loves and they’re not shy about expressing it, but they both felt like that was an important pizza and still relevant.”

You name it, and CPK can put it on a pizza or a pasta or in a salad, Pszybylski said. The reverse became true in March when CPK began putting pizza on macaroni and cheese as a hidden item at the bottom of the online order menu.

It came about through TikTok. In July, a customer ordered CPK mac and cheese for home delivery and only received cheese sauce. She documented the experience in a video that went viral.

Thinking on its feet, CPK’s team responded by sending her a large gift basket and casting Pszybylski in a satirical training video with an apology.

“Mac It!” is “very CPK,” Pszybylski said. “It’s like, let’s go for it. Let’s shoot for the moon and let’s try different things.”

Innovation is what customers can expect throughout the anniversary year, according to Warne. He wants CPK to be as ubiquitous as barbecue chicken pizza has become.

“We’re as excited about the next 40 years and we’re proud of the last 40,” he said. “Culinary innovation, you’re going to see that in restaurants. Growing the concept in nontraditional ways is where you’re going to see a lot of news.

“I can’t tell you the whole thing, but that’s the teaser.”