The 24-year-old Norwegian, runner-up to compatriot Thor Hushovd (Garmin-Cervelo) 24 hours earlier, worked himself into a 14-man escape group on the 179-kilometre route as the Tour headed into Italy.
Boasson Hagen, who sprinted to victory on stage six for Team Sky's first Tour stage success, underlined his climbing talents this time when he made the decisive attack on the day's final ascent with 11km remaining, jumping past Sylvain Chavanel (QuickStep) and soon distancing all his rivals.
He reached the summit of the Cote de Pramartino with a lead of around 20 seconds and fearlessly negotiated the 8km descent to win from Bauke Mollema (Rabobank), who was 40 seconds behind in second, with Sandy Casar (FDJ) third, 10 seconds further back.
Saur-Sojasun's Jonathan Hivert had been Boasson Hagen's closest pursuer but he tumbled on the descent and then lost control further down, entering a roadside driveway.
Voeckler sees GC advantage cut
Meanwhile behind that drama, the absorbing battle for overall glory continued as reigning champion Alberto Contador (Saxo Bank-Sungard) attacked for the second day running on the final climb, taking Andy Schleck (Leopard Trek), yellow jersey holder Thomas Voeckler (Europcar) and others, including Cadel Evans (BMC Racing) and Ivan Basso (Liquigas), with him.
However this time the initial acceleration achieved little so Contador made another burst, seeking to eat into his rivals' advantage on the descent, as Boasson Hagen celebrated victory.
Contador followed compatriot Samuel Sanchez's (Euskaltel-Euskadi) wheel, but their overall rivals caught up in the finale to finish on the same time with the exception of Voeckler who lost 27 seconds after visiting the same roadside driveway as Hivert on the descent.
The latest standings see Voeckler's general classification lead over Evans trimmed to one minute and 18 seconds, with Frank Schleck at 1:22, Andy Schleck at 2:26, Sanchez 2:59 back and Contador at 3:15.
Rigoberto Urán finished eight seconds back on the Contador group but maintained his lead in the white jersey classification and 10th place overall.
Planning pays off
The stage belonged to Boasson Hagen however and he said: “It was a really great day today. I felt a bit disappointed yesterday to not win. I really wanted the win today and I really tried to get into the break. My team-mates did a great job to help me get into it.
"It was really hard to get away in the group as riders were chasing behind. Finally we made it away and I was feeling great the whole day.
"I really wanted to win so I got some extra power from that and also from all the Norwegian fans out on the course. It’s so nice to win.
"We did the route in training last month and I was thinking about this stage. I had a plan and everything was perfect.
"It was a really great feeling to go up the climb alone. Nobody followed me and I knew almost every corner on the way down and could just time-trial to the finish. It was quite technical and I didn't find it dangerous when I was alone. I didn't find it too hard.
"The race goes into some really hard stages now so I will try to save my legs for the time trial (on Saturday)."
Speaking after the stage a delighted Sport Director Sean Yates was full of praise for the young Norwegian and a determined display by the whole team.
He said: "That was a top notch performance from Eddy and from the team. He has the legs, he showed that yesterday. Today he won with his legs as well as his head.
"He was very motivated. After yesterday he was a bit peeved about the result but there was nothing he could do about that. But regardless today he made amends. It was a big result.
On the technical descent that saw Boasson Hagen force home an advantage, he added: "It was a pretty gnarly descent. The boys have done it in training. Edvald looked at it this morning on the laptop as we had a film. But Edvald is not afraid, he is a superb bike-handler. It was a formality for him once he got over the top they were never going to bring him back unless he crashed.
"Rigo got tailed off a bit on the descent, as did a lot of guys. It was then just a race to the finish. We knew [the final climb] was potentially going to be decisive, especially after yesterday’s events. At the end of the day it’s only seconds. He’s lost eight seconds on Taaramae but it was never going to be minutes. The only thing was going to be if someone fell off.
"The team worked better than anybody to be in that breakaway. They were the definition of a team, with everyone helping each other out and motivating each other. G was up there in an early break with Eddy that Garmin chased down for a long long time. But Eddy really had the bit between his teeth today and there was no stopping him basically."