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1
Preheat oven to 350u00b0F.
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2
Grease 3 large mugs that are oven safe. The ones I used are about 5-6 inches in diameter. You could use just regular cake pans if need be, but the mugs are more fun.
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3
In a medium bowl, pour boiling water over the cocoa, and whisk until smooth. Let mixture cool.
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4
Sift together flour, baking soda, baking powder and salt; set aside.
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5
Using a stand mixer, cream butter and sugar together until light and fluffy, about 3 minutes. Beat in eggs one at time, scraping down the bowl after each addition. Beat in vanilla.
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6
Add the flour mixture alternately with the cocoa mixture. Spread batter evenly between the 3 prepared mugs (about 3/4 of the way up, maybe less).
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7
Place three marshmallows in the middle of the cake batter. Place the mugs on a baking sheet; the marshmallow will spill over but it's okay, makes for a nice effect. You will want to put tin foil over the cake as the marshmallow will brown too much otherwise. Make sure you tent the foil as opposed to wrapping the cake pan in it. Loose is your friend (yes, that sounds bad).
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8
The baking time is going to vary based on your mug or pan size. If you are just doing cake pans, it will be about 30 minutes. My mugs were smaller, but much deeper and took about 50 minutes to bake. You just need to watch and see.
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9
Remove from the oven and push another three marshmallows into the cake. Either put the cakes under a broiler for a minute or two or use a culinary torch to melt the marshmallows a little. Serve warm.
