Preheat oven to 410°F and line 2 baking sheets with parchment paper.
Cream the cold cubed butter and sugar together using a mixer for about 4-5 minutes until light and fluffy.
Add eggs one at a time, mixing well after each addition. Add vanilla extract if using.
In another bowl, whisk together cocoa powder, cornstarch, baking soda, salt, cake flour, and all-purpose flour.
Slowly add dry ingredients to wet mixture and stir until well combined. Do not overmix.
Fold in the dark chocolate chips by hand.
Divide dough into rough 6 oz balls and place on baking sheets spaced apart.
Bake for 9-11 minutes until edges are set and centers remain gooey.
Let cool on baking sheet for at least 10 minutes before transferring to a wire rack to cool completely.
Notes
Store in an airtight container at room temperature for up to 4 days, or refrigerate for up to one week. For longer storage, freeze individually wrapped bars.