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.
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.
Cookies can be stored in airtight containers at room temperature up to 1 week.