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.