Preheat the oven to 350°F (180°C). Line the baking sheet with a silicone mat or parchment paper. Set aside.
Mix 2 tablespoon cornstarch with 4 tablespoon of water in a small bowl and set aside.
Sift the flour, cocoa powder, baking powder, and salt in a medium bowl. Set aside.
In a stand mixer fitted with a paddle attachment, cream the butter and sugar on medium to high speed until light and fluffy. Scrape down the sides and the bottom of the bowl as needed.
Add cornstarch mixture and vanilla, and beat until everything is combined. Scrape down the sides of the bowl.
Turn the mixer on low, add the dry ingredients and milk, and mix until just combined. Next, add the chocolate chips and mix to distribute thoroughly.
Place the dough onto a prepared baking sheet and divide it into two equal parts. Shape each dough into a log about 8-9 inches long and 2½ inches wide.
Keep both the logs on a baking sheet with at least 3-4 inches of space between them. If using a small baking sheet, bake one log at a time.
Bake for 20-25 minutes or until the top is firm to touch and a toothpick inserted into the center comes out clean.
Remove from the oven and allow to cool on the cookie sheet. Carefully transfer the logs to a cutting board while still slightly warm.
Cut the logs into even diagonal slices about ¾-inch wide using a serrated knife. Transfer the biscotti back onto a baking sheet, laying them flat with a bit of space between them.
Return the biscotti to the oven and bake them again at 350°F (180°C) for 15 minutes or until crisp.
Remove from the oven and allow them to cool completely on the baking tray.
The biscotti can be kept in an airtight container for two weeks.