Preheat the oven to 350°F Butter and flour an 11 x 15 x 2 inch metal baking pan or spray with baking spray. Set aside. [Don't use a smaller 9 x 13 inch pan. It won't hold the batter, toppings and icing]
Mix together the hot water, cocoa powder and espresso powder until dissolved.
Melt the semi-sweet chocolate with the heavy cream in the microwave until completely smooth.
In a mixing bowl, cream together the butter, granulated sugar, brown sugar and vanilla. Cream for 3-4 minutes until combined. Beat in the eggs one at a time. Stop to scrape the bowl and beater as needed.
Add the water-cocoa-espresso mixture to the mixing bowl. Continue to beat for another 1-2 minutes until the ingredients are combined.
Sift together the flour, baking soda and salt. Add to the mixing bowl alternatively with the buttermilk.
Add the melted semi-sweet chocolate. Beat the batter together making sure you stop to scrape the sides of the bowl periodically so all of the ingredients are fully incorporated.
Pour the batter into the prepared pan and spread from end to end. Bounce the cake pan a couple of times on the counter to remove any trapped air bubbles and settle the batter.
Bake for 30-35 minutes or until a toothpick inserted into the middle comes out with moist crumbs.
Remove from the oven and immediately arrange the mini marshmallows on top of the cake while hot.
Return to the oven for 1-2 minutes to allow the marshmallows to soften. [If you prefer the marshmallows to maintain their texture, skip this step]
Sprinkle 1 cup of chopped pecans over the marshmallows.
To prepare the icing, melt the butter, heavy cream and cocoa together in a sauce pan on the stovetop. Melt just until smooth. Remove from the heat and add the vanilla.
Beat in the powdered sugar. Add additional cream if needed to bring it to a "drizzle" consistency.
Pour the warm icing over the soft marshmallows and toasted pecans. Sprinkle finely chopped pecans on top to garnish if desired.
Store at room temperature.