Cream puff pastry is an emulsion, in which essentially incompatible ingredients -- butter, eggs and flour -- must be as thoroughly combined as possible. Cream puffs deflate when the ingredients are not combined at the right temperatures. At the beginning, make sure that simmering milk, water, butter and salt are kept hot over medium heat while you beat in the flour. You will know the mixture is hot enough if you see a thin film of cooked dough on the pan. This means flour and butter can combine thoroughly. Whether you keep the cooked dough in the pan and beat by hand or transfer the dough to the bowl of a stand mixer, make sure the dough is warm to the touch before you add eggs. Very hot dough will cook bits of egg prematurely, while cold dough will not absorb the eggs thoroughly. Keeping the first mixture hot and the second warm maximizes the ability of ingredients to combine, making the resulting puffs puffy.

Several stages of choux pastry-making require prolonged beating. This effort elongates gluten fibers in flour, incorporates butter and ensures the eggs combine thoroughly with other ingredients. While recipes may specify three to five minutes of beating of flour and liquids, the cooked film on the bottom of the pan and a smooth, elastic, only slightly glossy dough are the best signs that your mixture is well beaten. High gloss means the butter is not fully incorporated and the puffs will be greasy and flat. Adding eggs to warm dough changes the texture, and, as you begin to incorporate each egg, the whole mixture will look lumpy, clotted and wet. Beat until the dough returns to a smooth state, then add the next egg. Failure to incorporate all the eggs thoroughly makes the puffs streaky, uneven and often flat.

The two most frequent causes of flat cream puffs during or after baking are uneven oven temperatures and inadequate baking time. Recipes typically start baking at 425 degrees Fahrenheit, then lower heat to 350 F after 10 minutes. The initial heat forces rapid steam expansion needed for puffing, and subsequent moderate heat crisps the outer crust and dries the puff inside. An oven thermometer can ensure your oven temperatures are correct and may help you avoid hot or cold spots in your oven. Set a timer so you know when to reduce the temperature. Subsequent baking usually takes about 20 minutes, but can be slightly confusing. The outer crust should be medium-brown and dry-looking. Test for inner moisture by inserting a thin wooden or metal skewer in the top. When the skewer comes out dry, the puffs are done. If you have not made cream puffs before, making a couple of extra test puffs can prevent you from removing the whole batch too early.

Nothing is more discouraging than watching a tray of puffs flatten as they cool. The cause is almost always excess internal moisture. Set the puffs on a rack to cool and poke a small hole in the top of each one with a skewer to release excess steam that could cause them to collapse. A solid outer crust will keep these small vent-holes from deflating the puffs like pricked balloons. Cool them completely before frosting or filling.