How does butter create crispness?

Oooh..COOL question, reader Monica! Forms greased with butter create crispy edges by facilitating the transfer of heat from the oven to the batter. That’s the first part. We coat things like vegetables and potatoes with oil before we oven-roast them for the very same reason: the oil film speeds cooking, and faster cooking means less dehydration of the food. The added benefit is that the fat helps deliver more flavor to the taste buds.

The reason the narrow edges of a financier get crispy is that they essentially fry. Frying is also a drying process — a very quick-drying process — at least where the exterior (crust) of a food is concerned. So much heat gets delivered to the outside of, say, a doughnut, that it’s completely robbed of moisture (which escapes in the form of steam) and becomes brittle. That brittleness doesn’t last terribly long, unfortunately. Within a few hours that brittle crust is softened. Moisture (either from the air or from under the crust) or residual surface oil gets in there and softens the crust back up again. And that’s a pity, because there’s nothing better than the crispy crust on a doughnut…or a piece of fried chicken, or a beignet, or a funnel cake, or a corn dog, or…excuse me, I’m getting light-headed. I’d better stop.