If you are looking for a delicious gelateria in Varenna, La Passerella is my favourite. They have a small selection of flavours, but they taste fantastic. I always get the classic fruit flavours, lemon and strawberry, and as this is always my go-to gelato, I can say it's one of the best I had in Lake Como.
The best thing you can do is grab a gelato and go at the end of the small port in front of the gelateria (the one with a red railing). There is the perfect spot to eat a gelato while looking at Varenna and enjoy the sun. I loved it!