Preheat oven to 350 degrees F (175 degrees C). Grease a 9x11-inch baking pan.
Place hot cocoa mix and butter in a large microwave-safe bowl; microwave until butter is mostly melted, about 3 minutes. Stir well. Add sugar, eggs, and vanilla extract and mix thoroughly. Add milk; mix until batter is thick and creamy. Mix in flour.
Bake in the preheated oven in 5 to 10 minute increments until set and sticky in texture, 25 to 65 minutes. Let cool for 10 minutes before cutting into bars.