Preheat oven to 300°F. Place butter and chocolate in a large microwave-safe bowl. Microwave on HIGH until completely melted, about 1 minute, stirring every 20 seconds. Add granulated sugar, and stir until well combined. Cool 10 minutes. Add eggs, 1 at a time, and stir until blended after each addition. Fold in all-purpose flour and salt. Stir in chopped toasted pecans and vanilla extract.
Coat a 9-inch square pan with cooking spray. Line bottom and sides of pan with parchment paper, allowing 4 to 5 inches to extend over sides. Coat parchment paper with cooking spray. Pour cake mixture into prepared pan.
Bake in preheated oven until a wooden pick inserted in center comes out clean, 45 to 50 minutes. Cool in pan on a wire rack 30 minutes. Lift cake from pan, using parchment paper sides as handles, and cool completely, about 30 minutes. Cut into squares, and dust with powdered sugar before serving.