This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Cream puffs contain a higher percentage of fat and eggs than popovers, but the proportion of liquid to flour is the same in each. The fat, flour, and liquid are first cooked together until a stiff ball of dough is formed. They are sufficiently cooked when a metal spoon pressed down on the stiff mass leaves a sharp, clear, smooth imprint. Next the eggs are beaten into the batter.
In the stiff batter produced by cooking the fat, flour, and water only a partial oil-in-water emulsion is formed. By staining the fat with Scarlet R its distribution in the batter can be determined by a microscope. Some of the fat is rather coarsely emulsified, and a good deal is found in layers and pools. Over-cooking of the fat, flour, and water increases the quantity of fat not emulsified. As the egg is added the fat becomes better emulsified, the dispersed spheres becoming smaller. The layers and pools of fat mostly disappear. The secret, and therefore the most important factor, in mixing cream puffs in order to obtain good expansion during baking and a good cavity is to beat the batter while adding the eggs until it is quite smooth, with no suggestions of small lumpy spots. Perhaps this is the reason why some directions for mixing cream puffs advise adding the unbeaten eggs one at a time. Incorporating the eggs one at a time at least insures thorough blending of the first eggs added. However, the eggs can be mixed with the batter as easily or more easily after they are beaten. When the puffs are made in large quantity, more than once the recipe, it is better to cool the paste slightly before the eggs are added because the large amount does not cool as rapidly as a smaller quantity. Some recipes state to cool the dough mixture before adding the eggs. On the whole, much better class results have been obtained by adding the eggs to the dough while it is warm. The probable explanation for this is that the surface tension is lower at the higher temperature and the emulsion forms more readily.
If the proportion of water drops below a certain level the tendency for the fat to run from the dough while baking is increased. This may be due to the fat being more easily emulsified in the thinner batter.
Sometimes the batter is very thin after the eggs are added, and it spreads and does not hold its shape on the baking sheet. When baked, the puff expands in one layer, giving a puff without a bottom or with a layer across the bottom that is too thin to hold a filling. This thin batter can sometimes be partially remedied by cooking and stirring over hot water in a double boiler until the batter is stiff enough to hold its shape when dropped on the baking sheet. The batter should be stiff enough to hold its shape, yet soft enough to appear glossy, and if forced through a pastry tube should retain the imprint of the tube teeth. The formation of the thin batter is not always due to under-cooking, for the flour, fat, and water may form a stiff ball. The fat is emulsified thoroughly enough to prevent its oozing out of the batter, but something happens that prevents the batter from becoming viscous. Usually, when the egg is first added the batter becomes somewhat thinner, but after being mixed the batter becomes increasingly viscous, until incorporation of the egg is complete, just as mayonnaise becomes stiffer as larger proportions of oil are emulsified.
Steam is the leavening agent, and the expansion due to the steam produces the cavity in the puff. Eggs are necessary, for their expansion and coagulation during cooking forms part of the framework of the baked puff. If the number of eggs is reduced and the amount of fat reduced at the same time a fair puff may be obtained. If the number of eggs is reduced the fat is not sufficiently emulsified and oozes from the puff as it bakes. As a result the volume of the puff is poor. The fat may be omitted and the puffs are then similar to popovers. Cream-puff like popover batter can be beaten as much as desired without perceptibly toughening the finished product. The cooking of the flour with the water and fat coagulates a portion of the flour protein. The starch granules also swell as the flour is cooked with the liquid.
The puffs like the popovers must be baked until very firm or they will collapse after removal from the oven. The best baking temperature is from 220° to 240°C. Cream puffs may be baked as well by starting in a cold oven as in a preheated oven, provided the oven heats quickly enough to reach a temperature of 220°C. in about 25 minutes. The batter may stand some time before baking provided it is left in a bowl and covered to prevent evaporation.