A marshmallow is a soft, spongy confection made by aerating a hot sugar syrup with a whipping agent — gelatin in almost all modern sweets, or beaten egg white in older and French-style recipes — into a light, springy foam that sets pillowy and tender. Bland, sweet and vanilla- scented, it is a global pantry staple and the building block of s'mores, rocky road and hot- chocolate toppers, prized as much for how it toasts and melts as for how it tastes.