Line a 12-inch pizza pan with heavy-duty aluminum foil; grease foil and set aside. Cream butter; gradually add sugar, beating well at medium speed of electric mixer. Add egg and beat well.
Combine flour and next 3 ingredients; add to creamed mixture, mixing well. Add vanilla extract and mix well. Stir in 1/2 cup chocolate pieces and 1/4 cup chopped pecans. Spoon batter onto prepared pan, spreading to within 1 inch of edge. Sprinkle with coconut and remaining 1/4 cup pecans. Bake at 350 degrees F for 15 minutes. Sprinkle marshmallows and remaining 1/4 cup chocolate pieces on top. Bake an additional 5 to 7 minutes. Cool on a wire rack.
To serve, cut into wedges.