Puff Pastry

Puff pastry, also known as pâte feuilletée, is a flaky light pastry made from a laminated dough composed of dough (détrempe) and butter or other solid fat (beurrage). The butter is put inside the dough (or vice versa), making a paton that is repeatedly folded and rolled out before baking.

The gaps that form between the layers left by the fat melting are pushed (leavened) by the water turning into steam during the baking process. Source: Wikipedia