Preheat your oven to 350 degrees F and grease a 9x13 cake pan.
Start by making the cake batter. Add the cake mix, flour, and sugar to a bowl and whisk together.
1 box Devils food cake mix, 1 cup flour, ½ cup sugar
Stir in the sour cream, vanilla, eggs, water and vegetable oil. Whisk until the batter is thick and smooth. Pour the batter into the greased pan and spread the batter so it is even inside the pan.
1 cup sour cream, 1 tablespoon vanilla extract, 3 eggs, 1 cup hot water, ½ cup olive oil or vegetable oil
Bake the cake for 30 minutes. A toothpick inserted into the center of the cake should come out cleanly.
Sprinkle the marshmallows over the top of the hot cake, making an even layer. Place the cake back in the oven for 3 minutes to melt the marshmallows slightly.
1 package (10 ounces) mini marshmallows
While the cake cools, make the frosting. Add the butter to a small pot and melt over medium heat.
8 tablespoons butter
Whisk the milk and cocoa powder into the melted butter then bring the mix to a simmer. Simmer for one minute while whisking.
6 tablespoons whole milk, ¼ cup unsweetened cocoa powder
Pour the chocolate butter mixture into a medium-sized bowl then add the powdered sugar, vanilla, and salt. Use an electric mixer to beat the frosting until it is smooth and thick.
3 cups powdered sugar, 1 teaspoon vanilla extract, ¼ teaspoon salt
Spread the chocolate frosting over the top of the cake, coating the marshmallow completely.
Sprinkle the pecans onto the cake, if using, then slice and enjoy!
1 cup chopped pecans, toasted (optional)