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 . . . .