Spoon batter into 2 greased and floured aluminum foil-lined 9- x 9- x 2-inch square pans. Bake at 350° for 40 minutes or until a wooden pick inserted in center comes out clean. Remove from oven; sprinkle each cake evenly with 2 cups miniature marshmallows and 1 cup pecans. Return to oven, and bake 5 more minutes or just until marshmallows begin to melt. Remove from oven, and let cool on a wire rack.
Bring butter, milk, and cocoa to a boil in a large saucepan over medium heat, stirring until butter melts. Remove from heat; whisk in powdered sugar and vanilla. Drizzle evenly over cakes in pans. Let cool completely. Cut cakes into squares.