Which ingredient is essential for the flakiness of puff pastry?

The essential ingredient for achieving the flakiness in puff pastry is butter. Puff pastry is unique because it relies on a technique known as lamination, where layers of dough are alternated with layers of butter. When the pastry is baked, the water in the butter creates steam, which lifts the layers of dough and results in the characteristic flaky texture.

The high fat content in butter contributes to the tenderness and helps create the distinct layers that puff pastry is known for. As the pastry bakes, the fat acts as a barrier, preventing gluten formation which keeps the pastry light and airy.

Other ingredients like yeast, milk, and sugar play different roles in baking but are not key to the flakiness of puff pastry. Yeast is used primarily in bread-making for leavening, milk adds moisture and richness, and sugar typically contributes sweetness or browning, none of which are fundamental to the lamination process that defines puff pastry's unique texture.