Shake Shack opened its first District location in Dupont Circle four years ago. (Ricky Carioti/The Washington Post)

Jason McDonnell bought 30 shares of Shake Shack Friday morning and sold them 10 minutes later for a $100 profit.

Then he headed out the door to try his first-ever Shackburger.

“I figured I bought their stock and made some money, I might as well give them some business,” the 26-year-old said.

The New York-based burger chain made its stock market debut Friday morning, and shares quickly skyrocketed from an initial public offering price of $21 to over $50. (Shares were trading at about $48 when McDonnell made his purchase a little after 10 a.m.; he sold them for $51.)

By lunchtime, a crowd of more than 50 snaked through the Shake Shack in Dupont Circle. Many customers said headlines of Shake Shack’s public offering had lured them to the restaurant.

“This is the busiest I’ve ever seen this place — except  for the day it opened,” said Emily Roth, 26, who works nearby at the non-profit No Kid Hungry. “I’ve been seeing stories about the IPO, but I didn’t realize it was going to be so crazy here.”

McDonnell, for his part, said he heard about the Shake Shack IPO on television. Although he works as an apprentice building engineer by day, McDonnell said he keeps a close eye on the stock market.

“Jim Cramer [host of CNBC’s ‘Mad Money’] has been hyping it for three or four weeks,” he said, adding that he bought shares of Facebook for similar reasons a few years ago. “This morning I sat there hitting refresh on my phone for a good hour and half before I was able to get any shares.”

Further back in line, Jim Rozier said he was kicking himself for not buying stock in the company.

“If I’d known it was today, I would have definitely bought some shares,” said Rozier, 55, who was in town from Raleigh to visit his daughter. “I mean, I’m ready to buy a franchise. We just love this place.”

Shake Shack (which does not franchise its locations) arrived in the District four years ago. It has since grown to five area restaurants, including locations at Nationals Park, Union Station and Tysons Corner Center, and amassed a loyal following among its fans.

Matthew Shuman, who works down the street from the Dupont Circle location, said Friday seemed as good a day as any to try Shake Shack for the first time.

“I was reading about the IPO on Twitter just before I came here,” Shuman, 26, said. “I figured it was time to try the burger.”

And as for buying shares?

“I’ll think about that after I eat lunch,” he said.

As for McDonnell, he took his Shackburger and fries back to his office on K Street, where he ate lunch at his desk.

“It was amazing,” he said, adding that he typically eats a homemade salad with chicken and hard-boiled eggs for lunch. “I normally never, ever eat out but I’ll be going back there again.”