Preheat your oven to 400°F (200°C). Line two baking sheets with parchment paper.

In a large bowl, beat the cold butter cubes with granulated and brown sugars until just combined but still chunky (about 2 minutes). Do not over-cream; you want small butter bits for texture.

Add the cold eggs one at a time, mixing on low after each addition. Stir in the vanilla extract until just incorporated.

In a separate bowl, whisk together flour, cocoa powder, cornstarch, baking soda, and salt.

Gradually add the dry mixture to the wet ingredients, mixing on low until just combined. Do not overmix.

Gently fold in the semi-sweet chocolate chips and chopped dark chocolate until evenly distributed.

Divide the dough into 8 equal portions (about 1/2 cup each). Roll each into a ball, but do not flatten. Place 4 balls on each baking sheet, spaced well apart.

Bake for 12-15 minutes, until edges are set and tops are cracked but centers are still soft. Do not overbake.

Let cookies cool on the baking sheet for 10 minutes, then transfer to a wire rack. Serve warm, stacked on a plate.