Preheat the oven to 350°F. Line a 9 x 13 pan with parchment paper, allowing it to overhang on the sides.

In the bowl of a stand mixer fitted with the paddle attachment, cream the butter, brown sugar, and granulated sugar until light and fluffy, about 3 minutes.

Mix in the eggs and vanilla until combined. Scrape down the sides of the bowl as needed to ensure even mixing. Set aside.

In a medium bowl, whisk to combine the flour, cocoa powder, baking soda, and salt.

With the mixer on low, add the flour mixture a little at a time, scraping down the sides and bottom of the bowl as needed. Mix until just combined - do not overmix.

Fold in 2 cups of chocolate chips.

Press the dough into the prepared pan in an even layer. Tip: Use a sheet of parchment paper or lightly coat your hands with nonstick cooking spray to make pressing the sticky dough into the pan easier.

Sprinkle the remaining cup of chocolate chips over the dough.

Bake for 22-26 minutes, or until the edges are set and the center of the dough is barely set.

Allow the bars to cool completely in the pan before removing, slicing, and serving.