Preheat oven to 350 degrees and spray a 9x13-inch baking pan with baking spray.
In a large bowl, whisk together flour and sugar.
In a medium bowl, whisk together melted butter, cocoa powder, and Coke. Pour into bowl with flour and stir until mixed well.
Add buttermilk, eggs, baking soda, and vanilla and mix well.
Stir in mini marshmallows and pour into prepared pan. Bake for 35 to 40 minutes. Let cake cool for 10 minutes before frosting.
To make frosting, beat butter, brown sugar, and peanut butter with an electric mixer until blended.
Beat in the milk and then stir in the peanuts. Spread frosting on cake.
Place cake under the broiler for a minute or two, or until the frosting just starts to get browned.