It's great being an adult: You can have dessert first. And you'll want to every time you eat at H Burger, which makes liquid-nitrogen milkshakes so cold and creamy they're almost powdery, with a texture like that mouthful of snow you get when you follow a friend down a ski hill after a big dumping — though they come out to the tables with billows of nitrogen smoke rolling off the tops. The hazelnut-chocolate Nutella version is particularly delightful, sinfully decadent and topped with tiny marshmallows that crack on the teeth and dissolve instantly on the tongue. Richer still is the shake that combines chocolate and peanut butter; more refreshing is the crisp, fruity strawberry-mint rendition.