Preheat oven to 350°. Pour water to a depth of 1 inch in the bottom of a double boiler over medium heat; bring to a boil. Reduce heat, and simmer. Place first 3 ingredients in top of double boiler over simmering water. Cook, stirring occasionally, 5 to 6 minutes or until melted. Remove from heat; cool 10 minutes.
Whisk together 3 eggs, granulated sugar, and next 2 ingredients in a large bowl. Gradually whisk warm chocolate mixture into egg mixture; cool 10 minutes.
Grease a 13- x 9-inch baking pan with butter. Line bottom and sides of pan with parchment paper, allowing 2 to 3 inches to extend over sides. Grease (with butter) and flour parchment paper.
Sift flour, baking powder, and 1/2 tsp. salt in a bowl. Whisk into chocolate mixture. Pour batter into prepared pan, reserving 2/3 cup.
Whisk together pumpkin, next 4 ingredients, and remaining 1/2 tsp. salt; pour over brownie batter in pan. Top with reserved brownie batter, and swirl batter gently 3 times in 1 direction and 3 times in the opposite direction with a knife or the end of a wooden spoon.
Bake at 350° for 45 to 50 minutes or until a wooden pick inserted in center comes out with a few moist crumbs. Cool completely on a wire rack (about 2 hours). Lift brownies from pan, using parchment paper sides as handles. Gently remove parchment paper, and cut brownies into 24 squares.
To make ahead, refrigerate cooled uncut brownies, uncovered, overnight; cut into squares while cold.