Dissolve the cocoa powder and espresso powder in the boiling water in a small bowl.

Using a hand mixer (original recipe calls for using a stand mixer with paddle attachment but the hand mixer worked, too), beat the butter and sugar in a large bowl on medium speed until the mixture is creamy. Mix in the vanilla and salt then, reducing the mixer's speed, mix in the egg followed by the cocoa mixture. Add the flour in small batches and continue to mix until a smooth dough forms. Since I used a hand mixer here, I used my hands to press the dough together.

Turn the dough out on a piece of plastic wrap and pat into a square. Wrap the dough and refrigerate it for about 30 minutes.

Preheat your oven to 325°F. Divide the chilled dough into 24 pieces and form into balls. Using a flat-bottomed glass, press each ball into a disk and set on a parchment-lined baking sheet (you may need two baking sheets).

Combine the egg yolk and water and brush the tops of the disks. Sprinkle with the turbinado sugar and bake in the oven for 20-25 minutes. The original recipe called for baking these for 35 minutes but I think that's too long. Watch the cookies at the 20 minute mark. If they're dry, they're done.

Allow the cookies to cool on a wire rack before serving. May be stored in an airtight container for up to a week.