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. [Tip:A smaller 13 x 9 inch pan. It won't hold the batter, toppings and icing]
Mix together the hot water and cocoa powder until dissolved. Set aside.
Melt chocolate chips with heavy cream in a microwave safe bowl using the melt function until completely smooth.
To make the batter: In a large mixing bowl using an electric mixer, cream together butter, granulated sugar, brown sugar and vanilla. Cream for 2-3 minutes until fully combined. Add eggs one at a time stopping to scrape the bowl periodically.
Add the dissolved cocoa 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. (Tip: A whisk works perfectly for sifting)
Add the melted semi-sweet chocolate. Beat the batter for 1-2 minutes stopping scrape the sides of the bowl periodically.
Spread batter evenly into prepared pan. Bounce on counter a couple of times to settle remove trapped air.
Bake for 30-35 minutes or until a toothpick inserted into the middle shows moist crumbs.
Remove from the oven and immediately arrange mini marshmallows on top. Return to the oven for 1-2 minutes to allow the marshmallows to soften. Sprinkle with pecans reserving 1/4 cup for garnishing. Set aside while you prepare icing.
To prepare icing: Melt butter, heavy cream and cocoa together in a saucepan over medium-low. Melt just until smooth. Remove from heat and add vanilla. Mix well.
Use an electric mixer to gradually add powdered sugar. May add more cream to thin to drizzle consistency, if needed.
Pour warm icing over marshmallows and pecans. Sprinkle finely chopped pecans on top to garnish if desired. Set aside to cool.
May store leftovers tightly covered chilled or at room temperature.