01 -
Set your oven to 350°F and grease a 9x13 inch pan lightly.
02 -
Mix together flour, both sugars, cocoa powder, salt, baking soda, and baking powder in a big bowl.
03 -
To the dry mixture, add eggs, oil, vanilla, sour cream, and milk. Blend well, then slowly add boiling water while mixing on low. Kick it up to high speed for about a minute to make the batter nice and airy.
04 -
Pour that batter into your greased pan and bake for 38 to 45 minutes. Check with a toothpick—it should come out clean.
05 -
Take the cake out and sprinkle the mini marshmallows on the hot surface. Pop it back into the oven for 3-5 minutes at 350°F until they look a little puffed and gooey.
06 -
Before adding the frosting, make sure the cake is totally cool.
07 -
In a small pan, melt butter over medium-high. Whisk in cocoa powder and milk right away. Stir it often for 2-3 minutes, then take it off the heat and add powdered sugar and vanilla. Use a mixer to smooth it out.
08 -
Spread the warm frosting all over the cooled cake. Chill it for 30 minutes so the topping sets. Keep it at room temp until you're ready to dig in.