Coat a 6-in. springform pan with cooking spray and dust with flour; set aside. In a small bowl, cream butter and sugar until light and fluffy. Beat in egg and vanilla. Combine the flour, cocoa and salt; stir into creamed mixture just until blended. Stir in pecans.
Transfer to prepared pan. Bake at 350° for 30-35 minutes or until a toothpick inserted in the center comes out clean.
For frosting, in a small saucepan, combine the butter, milk and cocoa; bring to a boil. Remove from the heat; beat in the confectioners' sugar and vanilla.
Place springform pan on a wire rack. Immediately spread marshmallow creme over hot cake. Drop 2 tablespoons of frosting over creme; cut through with a knife to swirl. Cool completely.
Carefully run a knife around edge of pan to loosen; remove sides of pan. Spread remaining frosting over top and sides of cake. Sprinkle with pecans.