Preheat the oven to 350 degrees F. Grease 2 9" cake pans, line with parchment paper and grease paper.

Whisk the flour, baking soda, baking powder, and 1/4 teas salt together in a large bowl.

Whisk the chocolate, cocoa powder, and espresso powder into the boiling water, until smooth.

In the bowl of your stand mixer fitted with the paddle attachment, beat the butter and brown sugar until smooth. Add the eggs, one at a time, pausing to scrape down the sides as needed. Add the sour cream and vanilla and mix again.

Mix in the flour and chocolate mixture in 3 additions starting and ending with the flour. Scrape down the sides and mix a bit longer just to make sure everything is incorporated.

Divide into the cake pans and bake 25 to 30 minutes.

Cool on a wire rack for 10 minutes before turning out to cool completely.

In a bowl, sprinkle the gelatin over the 1/4 cup cold water. Allow to sit for about 5 minutes. Place in the microwave for 15 or so seconds, to dissolve the gelatin (edges will bubble). Stir in the butter, vanilla, and salt. Allow to cool til warm to the touch before folding in the marshmallow creme. Stir until smooth. Cover and allow to set in fridge for 30 minutes.

Meanwhile, make the frosting. Heat 1/2 cup cream and cocoa powder in a saucepan set over medium high heat. Whisk until smooth. Remove from heat and pour mixture over white chocolate, whisking until smooth. Allow to cool completely, about 30 minutes.

Whisk the remaining 1 cup heavy cream in the clean bowl of your stand mixer fitted with the whisk. Whip the cream with the chocolate mixture until soft peaks form, about 60 to 90 seconds on medium high.

Spoon the marshmallow filling onto one of the cake layers, Smooth with a spatula and top. Frost the cake and top with marshmallows and dust with additional cocoa if desired.

Slice and serve.