-
Preheat the oven to 350°F. Grease and flour a 9" x 13" cake pan.
-
To make the cake: Melt the butter, then stir in the cocoa and hot water.
-
In a separate bowl, combine the flour, sugar, soda, salt, and espresso powder. Pour the cocoa mixture over the dry ingredients, stirring to blend; the mixture will be a bit lumpy.
-
Add the sour cream, eggs, and vanilla, beating until nearly smooth; a few small lumps may remain.
-
Pour the batter into the pan. Bake for 30 to 45 minutes, or until a tester inserted in the center comes out clean. Remove the cake from the oven.
-
Remove the cake from the oven and wait one minute. Spoon on the marshmallow, wait another 2 minutes, then gently spread the marshmallow over the surface of the cake. Allow to cool to lukewarm, about an hour.
-
To make the chocolate ganache: Combine the chocolate and cream in a microwave-safe bowl, and heat for 60 to 90 seconds, until the cream is very hot. Stir until the chocolate melts, and keep stirring until the mixture becomes smooth.
-
Drizzle over the cake. Top with the diced nuts and coconut.
-
Store, well covered, on the counter for a day, or refrigerate for up to a week.
