Heat a saucepan to medium. Salt the pecans and add them in a single layer in the saucepan and toast 8 to 10 minutes. Watch them carefully so they do not burn.
Preheat oven to 350°-. Spray 15x10x1 inch jelly roll pan with non-stick spray. I also like to line it with parchment paper or aluminum foil for easier cleanup.
For the Cake
Melt 1 cup butter and 4 ounces of chocolate in the microwave for 1 minute, stir. Heat at 30-second increments. Remove and stir until melted and smooth.
Whisk sugar, flour, cocoa, beaten eggs, vanilla, and salt together. Pour chocolate mixture into dry ingredients and combine. Don't over mix. Pour batter into prepared 15x10x1 inch pan.
Bake at 350°F for 20 minutes. Remove from oven and spread marshmallows evenly on top. Bake an additional 8 to 10 minutes or until marshmallows are melted and toasted.
For the Frosting
Stir together first butter, cocoa, and milk in a saucepan over medium heat until the butter is melted.
Cook, stirring constantly, 2 minutes or until slightly thick. Remove from heat.
Whisk in powdered sugar and vanilla. Whisk until smooth. Drizzle warm chocolate frosting over warm cake.