I’ve mentioned before that my nieces have celiac disease, so whenever I see them, we have a gluten-free extravaganza. I don’t mean that we spend all our time eating — on my latest visit, they took me to their favorite rock-climbing gym! — but treats are definitely on the agenda. While I usually bring cupcakes from Erin McKenna’s bakery in New York, that doesn’t mean we don’t explore the options in Massachusetts, where they live. On my most recent visit, that meant discovering Jennifer Lee’s together…