Stir cocoa and espresso powders into the boiling water in a small bowl until dissolved; set aside. Meanwhile, put butter and granulated sugar in the bowl of an electric mixer fitted with the paddle attachment; mix on medium speed until creamy. Mix in vanilla and salt. Reduce speed to medium-low. Mix in egg, then cocoa mixture. Gradually add flour, and mix until a smooth dough forms. Turn out onto a piece of plastic; pat into a square. Wrap dough, and refrigerate until cold, about 30 minutes.Advertisement
Divide dough into 24 equal pieces. Roll into balls. Shape balls into 12-inch-long ropes. Twist each rope into a pretzel shape. Space 1 inch apart on baking sheets lined with parchment paper.
Preheat oven to 325 degrees. Whisk egg yolk with 1 teaspoon water in a small bowl. Brush cookies with egg wash; sprinkle with sanding sugar. Bake cookies, rotating sheets halfway through, until dry, about 35 minutes. Let cool on sheets on wire racks.