In a medium saucepan cook and stir unsweetened chocolate and butter over low heat until melted and smooth. Remove from heat; cool slightly. Meanwhile, preheat oven to 350 degrees F. Line a 13x9x2-inch baking pan with foil, extending the foil over edges of pan. Grease foil; set pan aside.
Stir sugar into chocolate mixture. Add eggs, one at a time, beating with a wooden spoon after each addition just until combined. Stir in vanilla. In a small bowl stir together flour and baking soda. Add flour mixture to chocolate mixture; stir just until combined. Stir in semisweet chocolate pieces. Pour batter into the prepared baking pan, spreading evenly. Bake for 20 to 25 minutes or until edges are set and center is almost set.
Meanwhile, transfer dulce de leche to a small microwave-safe bowl. Microwave on 100 percent power (high) about 1 minute or until softened, stirring once.
Remove brownies from oven and set pan on a wire rack. Immediately spoon marshmallow creme in mounds on top of hot brownies. Drop spoonfuls of dulce de leche between mounds of marshmallow creme. Let stand for a few minutes to soften. Using a table knife or narrow metal spatula, gently swirl marshmallow creme and dulce de leche together to marble. Sprinkle with pecans. Cool in pan on rack.
Using the edges of the foil, lift uncut brownies out of pan. Cut into brownies, wiping knife as needed between cuts. Serve brownies the same day they are prepared.