Image source: Shake Shack.

The poster child of the "better burger" boom is looking better to investors that were previously bloodied rare. Shares of Shake Shack (NYSE:SHAK) soared 9.5% last week after posting better than expected financial results and bumping its outlook higher.

Revenue soared 43% to $54.2 million, fueled by heady expansion and a robust 9.9% spike in comps. It's a welcome surprise. Shake Shack's comparable-restaurant sales have been buoyant in the chain's brief public tenure, skyrocketing 13.3% through 2015. However, investors were braced for mortality in 2016 when it initiated guidance back in February, calling for just a 2.5% to 3% uptick in comps for the year. The fear at the time was that Shake Shack's comps would meet gravity as more of its eateries outside of its high-volume base in New York City would plop into the comps base. A restaurant has to be open for at least two years to be included, and that finds Shake Shack's comps base growing from 13 to 20 outlets. That wasn't the case. Shake Shack's proving itself to be a very portable concept.

The news only gets better as we work our way down the income statement. Operating profit, adjusted EBITDA, and earnings all grew even faster than Shake Shack's top line. It didn't have to sacrifice margins for the sake of strong sales growth. Adjusted earnings soared 114% to hit $2.8 million or $0.08 a share. Wall Street pros were only holding out for net income of $0.05 a share.

Shake Shack has routinely landed ahead of analyst profit targets since going public at $21 just 16 months ago, but the uninspiring guidance offered earlier this year suggested that the days of brisk growth and bottom-line beats were fading in the rearview mirror that was 2015. Thankfully that hasn't been the case.

The secrets of its success

The chain credits part of its success to the debut of its fried chicken sandwich. Chick'n Shack went for a national rollout in January, giving patrons wanting something other than a burger something to sink their teeth into.

Shake Shack is also talking up its successful push into the West Coast during the quarter. It opened a restaurant in West Hollywood in March, calling it one of the most successful debuts in its 12-year history. It will be another two years before the restaurant is lumped into its comps base -- more than half of what are now 47 company-owned locations are in the same boat -- but it's another reason to be excited about Shake Shack's growth prospects.

The ceiling is rising here. Shake Shack now expects to own 16 company-owned locations this year, up from its initial goal of 13 debutantes. It's also naturally pushing its February guidance higher. Shake Shack is now targeting 4% to 5% in comps growth and between $245 and $249 million in revenue.

One final point worth emphasizing is that the stock is now cheaper -- at least on a forward earnings basis -- than it was before last week's surge. If you're left scratching your head at that observation, let's go to the tape. Shake Shack's stock may have risen nearly 10% last week, but analysts on Friday scrambled to raise their forecasts. Wall Street pros pushed their earnings per share targets 16% higher for 2016 and 13% higher for 2017. So, yes, a stock's valuation can dip even as its stock takes off. Shake Shack may have fallen out of favor given its initial lofty markup, but it seems to be doing everything right these days to become a market darling again.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.