We weren't convinced an egg yolk was necessary in our sablé recipes. But after a little experimentation, our minds are made up.
When we came across sablé recipes that called for a hard-cooked egg yolk, we were tempted to ignore this unlikely ingredient. What could it possibly do—aside from fleck our cookies with unwelcome bits of yolk? But when we actually gave the cooked yolk a try, we were surprised by the results.
We prepared one batch of sablé cookies with a raw yolk and compared it with another batch made with a hard-cooked yolk that had been pressed through a fine-mesh strainer.
The cookies prepared with the hard-cooked egg yolk were markedly sandier than those prepared with the raw yolk. Even better, we could detect no bits of cooked yolk, just rich flavor.
An egg yolk consists of about 50 percent water by weight. When boiled, the proteins in the yolk form a solid matrix that locks in that water, making it unavailable. The sandiness in sablé cookies comes from undissolved sugar crystals. By reducing the amount of water available to dissolve the sugar, the cooked yolk promotes a crystalline texture in the finished cookies. By contrast, the liquid in the raw egg yolk dissolved the sugar, making the texture of the cookies smoother.