Preheat the oven to 325°F (160°C). Line a 9-inch (23-cm) square baking pan with parchment paper, leaving an overhang for easy removal.
In a medium mixing bowl, whisk together the flour, cocoa powder, baking soda, and salt.
In a large mixing bowl, whisk the melted butter, brown sugar, and granulated sugar until well combined. Add the eggs and vanilla extract, whisking until the mixture is smooth and creamy.
Add the flour mixture to the wet ingredients. Switch to a spatula or wooden spoon and fold in the dry ingredients halfway.
Set aside a small handful of chocolate chips for topping. Add the remaining chocolate chips to the dough and continue folding until the dough is combined. A few streaks of flour are fine.
Transfer the cookie dough to the prepared pan. Use your hands to gently pat the dough into an even layer. Sprinkle the reserved chocolate chips over the surface and gently press them into the dough.
Bake the bars for 28 to 30 minutes, or until the edges are done and pulling slightly away from the sides of the pan. The center will look slightly underbaked but will set as it cools.
Let the bars cool completely in the pan before lifting them out using the parchment overhang. Slice into squares and serve.
Notes
For easier mixing, warm the eggs (in the shell) in hot tap water before adding them to the butter and sugar.
The dough is pretty thick and crumbly. This is normal. Instead of overworking it with a spoon, use your hands to gently fold it together at the end.
Use an oven thermometer for accuracy.
Storage
Store in an airtight container at room temperature for up to 4 days. You may want to place parchment paper between layers to prevent sticking if stacking.
For longer storage, freeze in a sealed container or zipper-top bag for up to 2 months, letting them thaw at room temperature before serving.