Earlier this week, I visited Ninja’s pop-up Double Stack restaurant, which takes the form of a giant air fryer. The restaurant’s hook is that everything on the menu is cooked in a Ninja Double Stack air fryer. (As I had to leave early and was sneaked out through the kitchen, I can confirm that the team of chefs were indeed cooking in a troupe of Double Stack air fryers.) If that immediately sets off alarm bells, and you imagine that the menu would be made up of breaded chicken goujons, chips and the like, which go in frosty from the freezer and come out golden and crispy, it’s probably a sign …