Preheat the oven to 350°F (175°C) and line two baking sheets with parchment paper.
Beat the softened butter with an electric mixer until smooth and creamy.
Add the granulated sugar and beat until well combined.
Mix in the brown sugar and continue beating until the mixture becomes light and fluffy.
Add the vanilla extract and mix until incorporated.
Add the eggs and beat until smooth and creamy.
Add the flour and gently mix until partially combined.
Add the cocoa powder, baking soda, cornstarch, and salt. Fold everything together with a spatula until a thick chocolate cookie dough forms. Do not overmix.
Fold in the chocolate chunks until evenly distributed throughout the dough.
Scoop the dough onto the prepared baking sheets, spacing the cookies about 2 inches apart. Press a few extra chocolate chunks on top if desired.
Bake for 10–12 minutes, or until the edges are set and the centers still look slightly soft.
Let the cookies cool on the baking sheet for 5 minutes, then transfer them to a wire rack to cool completely.