There are a few essential points to ensuring your chawanmushi turns out satiny smooth. The first is to mix the egg well and strain it. Bits of unmixed eggwhite will result in a lumpy texture. The second point is to avoid incorporating air and then removing any bubbles or foam that floats to the surface. Having air bubbles in the custard gives it a spongy texture. Finally, the cups need to be heated gently so that the eggs cook slowly and evenly. This is achieved by putting the cups in a double boiler and insulating them with a dishtowel.