When Liza Black found out Halo Top ice cream was on sale at ShopRite last month, she and her husband went on a mission. At one store, they bought four pints of the Peanut Butter Cup flavor. Then, standing in the freezer aisle of another ShopRite without a shopping cart, Ms. Black was stacking pints up against her chest. A nearby shopper commented to the couple that they must really like the brand. "She grabbed a pint and walked away before I could grab the entire freezer's worth," Ms. Black said.

At a time when it's tougher than ever for products to earn and keep a place in the highly competitive supermarket freezer case, pints of low-calorie, protein-packed Halo Top are selling out in some stores, even with relatively high prices and, for now, limited marketing.

"We can't keep Halo Top on shelves, whether it's in L.A., New York or Wyoming," said founder and CEO Justin Woolverton. His product, labeled as light ice cream and as a good source of protein, is sweetened with erythritol, organic cane sugar and organic stevia. Vanilla Bean, Halo Top's best-selling flavor, contains 60 calories and 6 grams of protein per half-cup serving. The same size serving of Häagen-Dazs has 270 calories and 4 grams of protein.

Eat the whole Halo Top pint? That's OK. It's a relatively guilt-free 240 calories. In fact, some varieties come with instructions reading "Stop when you get to the bottom."

Mr. Woolverton, 37, founded Eden Creamery in 2011 and got his product in store freezers under that brand in June 2012. In 2013, he changed the name to Halo Top.

There are plenty of other lower-calorie frozen treats. Arctic Zero calls itself the pioneer of "Fit Frozen Desserts," and its pints contain fewer calories than Halo Top. Lighter ice creams such as Edy's Slow Churned have been around for years.

But word of mouth has propelled Halo Top's loyal following. Sales in 2016 jumped about 2,500% from 2015, Mr. Woolverton said.

The brand's sales at retailers hit nearly $66.1 million in the 52 weeks ended Feb. 19, with more than 13.5 million pints sold at an average price of $4.89, according to IRI.

That's without traditional media spending. "You can make $100 go a lot further through, say, a Facebook ad where you can target demographically, psychographically, geographically, et cetera, than by taking out ad space in a newspaper," Mr. Woolverton said. "If you can focus on people who actually want to see your ads, everyone is happier."

Kelly Nolan, head coach at Orange-
theory Fitness Roscoe Village in Chicago, said she first learned about Halo Top from a paid post on Instagram about a year ago. She searched for a while, then found Halo Top at Whole Foods Market. "Ever since then, I went on a rampage," said Ms. Nolan. "It became a hunt to try to find those flavors."

For her, a half cup of Halo Top has become a guilt-free way to indulge. "It kind of takes the bad feeling out of it."

When Ms. Black, from the Bronx, found Halo Top on sale for $1.88 at ShopRite, she stocked up and posted a photo of her haul on Facebook. "We're still working through it," she said.

Halo Top handles its own social media outreach and digital ads, though as it grows it is preparing to look outside for help.

For now, its in-house marketing team goes directly to representatives at Facebook and Google, rather than working with a main ad agency or media agency.

"We cringe at the thought of giving canned, corporate answers to everything. It's so boring," Mr. Woolverton said. Halo Top once tried having agencies design ads and social media posts. That lasted about a week. "We came to the conclusion that no one could get our voice down as authentically as we can, so we decided to pull it back in-house," he said.

Halo Top's Twitter feed, with nearly 20,000 followers, is peppered with pop culture references, emojis and free-product giveaways. So is its Facebook page, where more than 400,000 like the brand. Then there are the thousands of fan posts tagged #halotop on Instagram, where the company has more than 325,000 followers.

Mr. Woolverton said he aims to keep thinking like a "small brand at heart." Social and digital outreach "will almost certainly always be our main spends." Even so, as the brand grows, he's considering a broader outreach plan including traditional methods.

"We're about to reach a certain 'saturation' point where we can do more traditional marketing—billboards, TV spots," he said. "The downside with these traditional channels is that it'll be harder to measure ROI, but now that people who see the ad will actually be able to go buy Halo Top, it's an area we have to explore."