Passengers can’t get enough of Delta Air Lines’ in-flight snack of Biscoff, the commercial version of this Belgian confection. We took it to even greater heights. Speculoos are Belgian cookies with a crisp, light, open-crumbed texture and a blend of caramelized sugar and warm spice flavors. To achieve the appropriate texture, we rolled the dough thin so it would bake up dry and crisp, used only enough sugar to lightly sweeten the dough since sugar is hygroscopic and makes cookies moist, and…