Heat oven to 350°F Grease and flour 13x9” baking pan.
For cake, in large bowl, beat together butter, sugar, and cocoa until smooth and creamy. Add eggs, one at a time, beating well after each addition. Add flour, pecans, and vanilla. Mix just until blended. Pour into pan.
Bake 30 to 35 minutes or until wooden pick inserted in center comes out clean. Cool on wire cooling rack.
For icing, combine butter, sugar, marshmallows, cocoa, and milk in medium saucepan. Cook over medium-low heat just until marshmallows are melted, stirring frequently. Drizzle over cake.
Sprinkle with pecans and cool completely. Garnish with whipped cream and cherries.