Preheat oven to 325°F. Line an 8-inch square metal baking pan with foil; grease.
Microwave chocolate and 2 Tbsp. water on medium for 2 minutes in a microwavable bowl; stir until smooth. Stir in pumpkin puree.
With an electric mixer, beat eggs, sugars, vanilla and salt on high until thick, about 2 minutes. Add pumpkin mixture on low. Stir in cocoa and then flour until blended.
Scrape batter into prepared pan. Bake until a tester comes out clean, about 30 minutes. Let cool on a rack for 1 hour.