Preheat the oven to 350º. Line a muffin tin with paper liners.

Place about 2 inches of water in a saucepan, and bring to a boil. Reduce the heat to a simmer.

In a stainless bowl, measure together the butter, chocolate and cocoa powder. Set the bowl over the simmering water, and heat, stirring frequently, until melted and smooth. Set aside to cool until just warm to the touch.

In a small bowl, measure together the flour, baking soda and baking powder.

In a medium-size bowl, whisk together the eggs, sugar, vanilla and salt. Whisk until fully incorporated.

Add the cooled chocolate mixture, and whisk to combine.

Add about 1/3rd of the flour mixture, and stir to combine. Stir in the sour cream followed by the remaining flour mixture. Stir until all of the ingredients are completely incorporated.

Using a portion scoop, divide the batter evenly between the prepared muffin cups. Bake until a toothpick inserted into the center of the cupcake comes out clean, 18 to 20 minutes. Allow to cool.

Using a serrated knife, slice off the rounded tops of the cupcake, leaving at least 1/3 inch of undisturbed cake around the edges. Using small scoop, remove the center of the cupcakes and fill with Gooey Chocolate Ganache. Replace the rounded tops of each cupcake to seal the ganache inside

Pipe or spread Marshmallow Frosting over the top of each cupcake, and garnish with mini chocolate chips or chocolate shavings.