Combine 1/2 cup butter and brown sugar in a medium saucepan over medium heat. Bring to a boil; boil 3 minutes, stirring constantly. Stir in pecans. Working rapidly, drop individual pecan halves onto wax paper; let stand until cooled and firm. Set aside 16 pecans for garnish; coarsely chop remaining pecans.
Melt 1 cup butter in a large saucepan over medium heat; stir in 1/2 cup cocoa until combined.
Remove from heat; whisk in eggs and granulated sugar.
Combine flour and salt; gradually stir into chocolate mixture. Stir in vanilla and chopped sugar-coated pecans.
Pour batter into a greased 13- x 9-inch pan.
Bake at 350 for 35 minutes or until a wooden pick inserted into center comes out clean.
Remove from oven; immediately sprinkle marshmallows over hot cake.
Beat 1/2 cup melted butter, remaining 1/2 cup cocoa, and milk with an electric mixer at medium speed.
Add powdered sugar; beat at low speed until smooth.
Spread immediately over marshmallows.
Garnish with reserved sugared pecans. Cool cake in pan on a wire rack.