Preheat oven to 350 degrees. Line a 9X13 baking dish with foil or parchment paper and spray with a non stick cooking spray. Combine cocoa and melted butter in bowl and mix with an electric mixer.
Then add flour and sugar and continue to beat together. Next add eggs and vanilla. Mix until combined, but don’t over mix. The batter will be thick.
Next pour into prepared baking dish and spread evenly. The batter will be thick, so this may take a moment.
Cook for 25 minutes, remove brownies, top with marshmallows, then pretzels.
Put back in the oven for 5 minutes. Then turn the broiler on and let them brown just a little. DO NOT WALK AWAY this will only take a moment.
Let cool and cut and serve.
Notes
You may need to check brownies at 25 minutes to test to see if they are close to done. All ovens vary.