The hosts won Tuesday's first T20 in Southampton by 119 runs and followed it up on an overcast evening with a 14th consecutive victory over Sri Lanka in the format.
Set a target of 146, after winning the toss and opting to bowl at a murky and overcast Sophia Gardens, England had marched to 0-48 after four overs but openers Aneurin Donald (21) and Buttler (30) then walked in quick succession to make it 2-55.
Big-hitting captain Harry Brook smashed three sixes in one over but went for 23, well caught by Eshan Malinga off a Wanindu Hasaranga delivery, before Tom Banton steadied the ship with a top-scoring 52 not out from 24 balls as England cantered towards another easy win with 47 balls and 7.5 overs to spare.
Sri Lanka posted 9-145 from their 20 overs, an improvement on their previous 135 all out in 19, with opener Pathum Nissanka hitting 37 from 21 balls.
They had carved out a promising 0-56 but lost momentum with Jamie Overton taking the first wicket with a leaping catch off player of the match Dawson, who took his second wicket with the next ball of the sixth over and ended with figures of 3-32.
Jofra Archer (2-34) returned for his 50th T20 cap after being rested for the first T20 of the series, with fellow-seamer Josh Tongue dropping out, and took two wickets in the final over with Dushmantha Chameera bowled middle stump for a duck and Tharindu Rathnayake caught by Brook.
Sonny Baker had another excellent evening with the ball, finishing with 1-15 from his four overs with a wicket maiden in the 19th.
Donald, playing on home Welsh soil after making his debut in Southampton, dropped Dasun Shanaka but made up for it with a catch three deliveries later for Dawson's third wicket.
The final T20 of the three-match series is at Old Trafford on Saturday.
