Preheat over to 350 degrees.

Line a 12-count muffin tin with cupcake cups. Reserve.

In the bowl of a stand mixer, combine the butter, the sugar, the cocoa powder and the boiling water. Beat on low until smooth and the sugar is dissolved.

In a separate bowl, combine the flour, the salt, the baking powder and the baking soda. Reserve.

In a third bowl, beat the egg and add the sour cream and the vanilla extract. Whisk until smooth. Reserve.

With the mixer on low, add 1/2 the dry ingredients to the butter/sugar/boiling water mixture. Then, add the egg/sour cream and finish with the remaining of the flour. Mix just until the flour is incorporated. Do not overmix or your cupcakes won't be moist and pillowy!

Pour the batter into the cupcake cups, about 2/3 full.

Bake the cupcakes for 20 to 25 minutes or until a toothpick inserted in the center of a cupcake comes out clean.

Cool in the pan for 5 minutes. Remove from the pan and cool completely before frosting.

In the bowl of a stand mixer, combine the butter and the marshmallow fluff. Beat until creamy and smooth.

Add the vanilla extract and the heavy cream and beat until incorporated.

With the mixer on low, slowly add the confectioners sugar. Once incorporated, turn the speed up and beat for 1 minute, until light and fluffy. Add more powdered sugar if frosting is too thin or more cream if frosting is too thick. Add a pinch of salt if frosting is too sweet.

Frost cooled cupcakes (I used a Wilton 12 tip) and decorate with the eyes (and mouths).