Simple and sweet, this tasty treat is perfect for getting kids involved in baking.
Make the cake: Preheat oven to 350 degrees. Line cupcake tins with cupcake liners.
Whisk cocoa powder, flour, sugar, baking soda, baking powder, and salt in a large bowl. Stir in eggs, warm water, milk, sour cream, butter, and vanilla, mixing until smooth.
Fill each muffin cup halfway (about 2 ounces each). Bake until a tester inserted into the center comes out clean, 15 to 18 minutes.
Let cake cool slightly and change the oven to broil.
Make the topping: Place one marshmallow vertically in the center of each cupcake. Put the cupcakes back into the oven (careful not to knock any marshmallows off) and watch them closely, as the marshmallows will roast in a matter of seconds.
When the marshmallows appear golden brown on top and puffed slightly, take the cupcakes out of the oven and carefully press the marshmallows down, using your hand or a spoon, to cover the whole top of the cupcake.