Preheat oven to 325° F. Line a 9 x 13 pan with parchment paper or foil and spray with cooking spray.
Whip eggs until light and very bubbly.
Add sugar and vanilla and whip to ribbon stage — it should leave trails as you mix or when you lift the whisk that take a second or two to melt back into the mixture.
Melt chocolate chips and butter together in the microwave for a minute at a time until smooth. Mix into the eggs and sugar.
In a separate bowl, whisk together flour, cocoa, salt, and baking soda.
Gently mix dry ingredients into wet ingredients just until most of the flour is incorporated. There will still be some streaks of flour throughout.
Spread batter into prepared pan. It will be very thick.
Bake at 325° F for 30-40 minutes until toothpick inserted in center comes out with a few sticky crumbs, not batter.
Enjoy!