Mississippi Mud Cake

1 cup (2 sticks) unsalted butter, melted

2 cups granulated sugar

1/2 cup unsweetened cocoa powder

4 large eggs, beaten

1 teaspoon vanilla extract

1 1/2 cups all-purpose flour

1/2 teaspoon salt

3 cups mini marshmallows

1 cup chopped pecans or walnuts (optional)

For the Chocolate Frosting:

1/2 cup (1 stick) unsalted butter, melted

1/3 cup milk

1/4 cup unsweetened cocoa powder

4 cups powdered sugar

1 teaspoon vanilla extract