Two 12 by 17-inch baking sheets, lined with parchment paper
Place the yeast, warm water and 1/4 teaspoon sugar in a large bowl. Stir with a fork to dissolve the yeast and allow it to stand for about 3 minutes.
Whisk the unbleached flour, cocoa powder, the remaining 1/3 cup sugar and the salt together in a medium bowl. Set them aside.
Using your hand, stir the coffee, the egg yolk, and the butter into the dissolved yeast. Gradually add the flour mixture, stirring until a shaggy mass forms and all of the flour is moistened.
Move the dough to a lightly floured surface and knead for about 7 to 8 minutes, until it is silky smooth and elastic. This dough should be nice and moist, so add flour sparingly as you work. Shape the dough into a loose ball cover it with plastic wrap and let it rest for about 10 minutes to relax the gluten strands.
Flatten the dough and stretch it gently with your fingers to form a rectangle about an inch thick. Spread the chocolate chips evenly over the rectangle. Fold the dough into an envelope and knead gently for 2 to 3 minutes, until the chips are well distributed. The dough should be soft, smooth, and springy. If the dough resists, let it rest for 5 minutes and then continue kneading it. Some of the chips may pop out of the dough, but they can easily be incorporated again after the first rise when the dough has softened.
Shape the dough into a loose ball and place it in a lightly oiled bowl. Turn to coat the top of the dough with oil, and cover the bowl tightly with plastic wrap. Let rise at room temperature (75 to 77 degrees F) until the dough has doubled in volume, about 2 hours.
Line two 12 by 17-inch baking sheets with parchment paper. Gently pour the dough onto the floured work surface pressing any loose chocolate chips into dough. Flour your hands lightly divide the dough into 12 equal pieces (each weighing about 2 1/2 ounces). Shape the pieces into rolls and place 6 on each prepared baking sheet leaving several inches between the rolls so they won't grow together as they rise. Cover them loosely with oiled plastic wrap and let them rise at room temperature until doubled in volume, about 45 minutes to 1 hour.
While the bread is proofing make a cornstarch wash: Put the cold water in a small saucepan and whisk in the cornstarch. Bring to a boil, stirring frequently until it thickens. Remove from the heat and cover to keep a skin from forming. Set aside to cool.
About 15 minutes before you're ready to bake, place one oven rack in the top third of the oven, and another in the bottom third, and preheat the oven to 400 degrees F.
When the buns have doubled, use a pastry brush to paint them with a thin coating of the cornstarch wash. Brush gently so you don't deflate them. Place one pan on each oven rack. Using a plant sprayer, immediately mist the top and sides of the oven 6 to 8 times and quickly close the oven door. Repeat this misting procedure two more times at 1-minute intervals.
Bake for 10 minutes, then reduce the oven temperature to 350 degrees F and rotate the pans from top to bottom and front to back to ensure even baking. Bake for 10 to 15 minutes longer, or
until the tops of the buns feel firm but not hard when you press them slightly, and the bottoms are very lightly browned. These rolls should have a thin soft covering, not a hard crunchy crust. Remove from the oven and use the pastry brush to paint the top of each bun quickly and evenly with the cornstarch wash. Transfer the rolls to a rack and allow them to cool before serving. These are best eaten the day they are baked.