Preheat your oven to 350°F (175°C). Lightly grease a 9-inch round cake pan or line it with parchment paper.
In a large mixing bowl, beat the softened butter, brown sugar, and granulated sugar together until light, fluffy, and pale, about 3-4 minutes.
Add eggs one at a time, beating well after each addition. Stir in vanilla extract until smooth and glossy.
In a separate bowl, whisk together the all-purpose flour, unsweetened cocoa powder, baking soda, and salt.
Gradually add dry ingredients to the wet mixture, mixing on low speed or folding gently until just combined. Avoid overmixing.
Fold in the chocolate chips gently, reserving some to sprinkle on top if desired.
Transfer the dough to the prepared pan, spreading evenly. Sprinkle reserved chocolate chips on top. Bake for 25-30 minutes until edges are set and a toothpick comes out with moist crumbs.
Cool in pan for at least 15 minutes, then transfer to a cooling rack. Serve warm or at room temperature.