01 -
Preheat oven to 350 degrees Fahrenheit. Lightly grease a 9x13 inch baking pan.
02 -
In a large mixing bowl, combine the all-purpose flour, granulated sugar, light brown sugar, cocoa powder, baking powder, baking soda, and salt.
03 -
Add the oil, eggs, vanilla extract, milk, and sour cream to the dry ingredients and mix until fully combined. Reduce speed and carefully pour in the boiling water. Beat on high speed for 1 minute to incorporate air into the batter.
04 -
Pour the batter into the prepared baking pan. Bake for 38-45 minutes, or until a toothpick inserted in the center comes out clean.
05 -
Remove the cake from the oven and sprinkle mini marshmallows evenly over the top of the hot cake. Return the cake to the oven for 3-5 minutes at 350 degrees Fahrenheit, or until the marshmallows are slightly puffed and melted.
06 -
Allow the cake to cool completely before adding the frosting.
07 -
Melt the butter in a small saucepan over medium-high heat. Add the milk and cocoa powder, whisking well. Cook for 2-3 minutes, stirring constantly. Remove from heat and stir in the vanilla extract and powdered sugar. Use an electric mixer to create a smooth frosting.
08 -
Drizzle the warm frosting evenly over the cooled cake. Refrigerate for 30 minutes, or until the frosting is set. Store the cake at room temperature.