Preheat oven to 350°. Grease a 15x10x1 inch jelly roll pan.
Place pecans on a baking sheet and bake for 8-10 minutes until toasted.
In a large microwavable bowl, microwave 1 cup butter and semisweet chocolate for 1-2 minutes or until melted, stirring every 30 seconds.
Whisk sugar and next 5 ingredients into chocolate mixture.
Pour batter into pan and bake for 20 minutes.
Remove from oven and sprinkle with marshmallows. Bake for 8-10 more minutes or until golden. While marshmallows bake, prepare chocolate icing.
In a medium saucepan, combine 1/2 cup butter, 1/3 cup unweeentened cocoa and milk. Cook over medium heat, stirring contstantly for about 2 minutes after butter is melted until slightly thickened. Remove from heat.
With an electric mixer, beat in the powered sugar and vanilla at medium-high speed until smooth. Drizzle over warm cake and sprinkle pecans on top.