Adapted from HoneyB (see above) and allrecipes.com
1/3 cup butter, softened
2/3 cup white sugar
2 large eggs
1 1/2 t vanilla extract
1/3 cup unsweetened cocoa powder (I used natural, Scharffen Berger)
2 t baking powder
3/4 cup chocolate chips, chopped roughly (or use miniature)
1 egg yolk, beaten
1 T water
Preheat oven to 375 degrees F. Line baking sheet with parchment or silicone.
In a large bowl, cream together the butter and sugar until smooth. Beat in the eggs one at a time, then stir in the vanilla and sea salt. Whisk together the flour, cocoa, instant espresso and baking powder; stir into the creamed mixture until well blended. Dough will be stiff, so mix in the last bit by hand. Mix in the chocolate chips.
Divide dough into two equal parts and shape them into 9x2x1 inch logs directly on the baking sheet, about 4 inches apart. Brush them with a mixture of water and yolk. Sprinkle with sea salt.
Bake for 20 to 25 minutes in the preheated oven, or until firm. If your oven is uneven (mine is) be sure to rotate front to back halfway through. Cool on baking sheet for 30 minutes. Reduce heat to 325 F.
Using a serrated knife, slice the loaves diagonally into 1 inch slices. Return the slices to the baking sheet, placing them upright. Bake for 20-30 minutes, or until dry. Cool completely and store in an airtight container.