Preheat oven to 410° degrees F. Line 2 baking sheets with parchment paper or silicone liners; set aside.
In the bowl of a stand mixer, cream together cubed butter pieces and granulated sugar on low speed for 30 seconds. Increase speed to medium and beat for another 30 seconds, then increase to high speed and beat for a final 30 seconds, scraping down the sides of the bowl as needed.
Add in the eggs, one at a time, beating well after each addition, followed by the vanilla, if using. Add in the dark cocoa powder, cornstarch, baking soda, salt, cake flour, and all-purpose flour and mix on low speed until just combined. Mix in the dark chocolate chips until fully incorporated.
Refrigerate the dough for 15 minutes before baking.
Using your hands, scoop out rough 6-ounce portions of cookie dough (using a digital cooking scale for accuracy is best). Roughly form the dough into a ball shape - it should be rougher textured, not at all like the smooth compacted cookie dough balls we're used to (see photos in post for an idea). Place 3" apart staggered on a lined baking sheet with 4 cookie dough balls per sheet.
Bake one sheet at a time in the center rack of the oven for approximately 8-12 minutes, checking at the 8-minute mark. Cookies should appear dry and dull on top and look mostly done with slightly gooey centers; increase baking time if needed. Allow cookies to cool on the baking sheets for at LEAST 30 minutes before serving, as they will continue to finish cooking and setting on the baking sheet after being removed from the oven and are quite delicate straight from the oven.