Tàng, 烫: scald.
Miàn, 面: flour.
Tang mian is roux, made by cooking flour in bubbling hot butter.
Tang mian cake has the fluffiness of chiffon cake and the butteriness of butter cake. It has the best of two cake worlds but that's not all. It is smooth, smoother than chiffon or butter cake could ever be.
Some people call tang mian "cooked dough" instead of "roux". And the cake is sometimes called "黄金蛋糕" or "golden sponge cake". Hey, a rose by any other name . . . .
Mixing bubbling hot butter with flour intensifies the butter aroma significantly. If butter were just melted and added to the batter, or it's just beaten till fluffy, the cake would taste less buttery. Even beurre noisette isn't as good.
Cooking flour in hot butter makes the cake smooth. Why? Because when flour is coated with fat, it doesn't form gluten when it's mixed with non-fat liquids. No gluten is good news for cakes because gluten has a rough texture. In fact, pure gluten looks quite like sea sponge.
Once the roux is made, it's transferred into a bowl and mixed with cold milk. The milk is absorbed by the starch in the flour, creating a thick paste that's very hot. Unlike gluten which is rough, thickened starch has a smooth texture. That's why tang mian cake is smoother than other types of cake (when it's done right).
After the edge of the heat cools down, a small amount of egg white is added to the thick paste. When this is done is critical to the success of the cake. Why? Because the timing affects the consistency of the starch. If the starch is too thick/thin, the cake won't rise well. If the starch is just right, the cake will be very smooth. You know how good wine feels smooth in the mouth? Hey, cake can be smooth too!
After combining the roux with egg white, along with vanilla extract and salt, I add egg yolks. These are added one at a time so that they are mixed but not emulsified with the starch. The yolk batter mustn't stick to the whisk. Otherwise, it's too thick and the cake is doomed.
The bulk of the egg whites in the recipe are whisked, separately with sugar and cream of tartar, till firm peak stage.
If the batter is bubbly after whisked egg whites are added to the yolk mixture, it's too thin. Thin batter allows air bubbles to escape easily, so the cake won't rise well. It also allows air bubbles to expand easily when heated, so the cake will be pockmarked.
The batter is poured into the cake pan, slowly and from about 30 cm high. This releases big air bubbles in the batter, so the crumb will be fine and smooth.
If the batter falls in clumps when it's poured, it's too thick. Big air bubbles remain trapped in the batter, turning into unsightly holes in the cake when the baking is done. If the batter is too thick to rise well, the cake will be short and dense.
What makes the batter too thick? The roux is too hot when egg white is added, or the meringue is overwhisked. And vice versa would make the batter too thin.
After baking, the cake must be inverted whilst it's cooling down. If it's underbaked, it'll fall out of the pan and you can wave goodbye to its fluffiness. If the cake is overbaked, it'll be too dry. When the cake is baked just right, it's delightfully moist and fluffy . . . . Oh hang on . . . I think there's someone at the door.
Knock, knock! Who's there? Tang mian! Tang mian who? Timing is everything!
Want to get the timing right so that your cake is fluffy, moist, smooth and buttery? Here's how: