01 -
Set the oven to 350°F and lightly grease a 9x13-inch baking dish.
02 -
Toss your flour, both types of sugar, cocoa, salt, baking powder, and baking soda into a big mixing bowl.
03 -
Pour in the oil, eggs, vanilla, sour cream, and milk. Stir until smooth. Slowly stream in the boiling water while mixing on low. Turn the speed to high for 1 minute to fluff it up.
04 -
Pour the batter into the greased pan. Bake for 38 to 45 minutes—insert a toothpick in the center, and if it comes out without batter, it's done.
05 -
Straight from the oven, scatter the mini marshmallows across the top. Slide it back into the oven for 3 to 5 minutes at 350°F until they're puffed and sticky.
06 -
Wait for the cake to cool completely before slathering on the frosting.
07 -
Melt the butter in a little pot over medium-high heat. Mix in the cocoa and milk, whisking non-stop. After about 2-3 minutes, pull it off the heat. Add in the vanilla and powdered sugar, then use a mixer to smooth it out.
08 -
Spread the warm frosting evenly across the cooled cake. Chill it for 30 minutes to firm up the frosting. Keep at room temp afterward.