Adjust oven racks to upper-middle and lower-middle positions and heat oven to 300 degrees. Line 2 rimmed baking sheets with parchment paper. Whisk flour, cocoa, baking powder, baking soda, and salt together in bowl; set aside.

Microwave 6 ounces (170 grams) chocolate, oil, and butter in medium bowl at 50 percent power, stirring halfway through microwaving, until melted, about 3 minutes. Whisk milk into chocolate mixture until combined.

Using stand mixer fitted with whisk attachment, whip sugar and eggs on medium-high speed until very thick and pale, about 4 minutes. Remove bowl from mixer. Add melted chocolate mixture and whisk by hand until uniform. Fold in flour mixture with rubber spatula until thoroughly combined and no dry pockets remain. Fold in remaining 4 ounces (113 grams) chocolate.

Using 1-tablespoon measure or #30 scoop, scoop 10 heaping-tablespoon portions of batter onto each prepared sheet (you should have 20 cookies total). (Use soupspoon to help scrape batter from tablespoon, if necessary.) Bake until cookies are puffed and covered with large cracks (cookies will look raw between cracks and seem underdone), about 16 minutes, switching and rotating sheets halfway through baking.

Let cookies cool completely on sheets, about 30 minutes. Slide thin spatula under cookies to remove from sheets. Serve. (Cookies can be stored in airtight container at room temperature for up to 3 days.)