Preheat oven to 350°F/175°C. Prepare a baking sheet by lining it with parchment paper.
Place the butter in a mixing bowl. Use either a handheld or stand mixer on medium speed and beat until light and fluffy. Add the sugar and beat again until light and fluffy. Add the flax, cornstarch, milk, and extract. Beat again to combine.
Add the flour, baking powder, and cocoa powder to the bowl and beat on low speed to combine. Stir in the chocolate chips.
Transfer the dough to the prepared baking pan. Divide the dough in half and pat it into two logs, approximately 9x2 inches. Each loaf should be about ¾ of an inch tall. Sprinkle with coarse sugar.
Bake for 20 to 25 minutes. Then remove from the oven and allow the biscotti loaves to cool about 10 minutes.
Use a spray bottle to spray room temperature water over the top of both loaves.
Reduce oven to 300°F/150°C and place the loaves on a cutting board and use a serrated knife to slice the loaves into approximately ¾-inch slices. Place these slices back on the prepared pans and bake for 10 minutes. Remove from the oven and turn the slices over and bake for another 10 minutes.
After the biscotti are done, place them on a wire rack to cool completely.