In a small bowl, whisk together cocoa and hot water into a thick paste.
In a separate bowl, whisk together the flour, baking soda, baking powder, and salt.
Melt butter in a large saucepan over low heat, then add the sugar, vanilla and the cocoa mix. Stir well to combine. Remove from the heat and transfer to a large bowl. Add the eggs, then the sour cream and mix well.
Pour into the greased tin and bake for 60 minutes or until a toothpick in the centre comes out clean.
Allow to cool enough so you can handle the tin, then gently remove the cake. Transfer to a wire rack and cool completely.
Make the frosting: beat the butter until light and fluffy then add the icing sugar. Whip for about 2 minutes then add the marshmallow fluff beat for another two minutes.
Place the coconut in a food processor with a small amount of food colouring. Pulse a few times to mix completely so the colour is even.
To assemble: using a bread knife, level the cooled cake if it has domed at all during baking. Place cut side down on a serving plate and place strips of wax paper under the cake for easy clean up. Then cut the dome off the cake, about 1/3 of the way down.
Carefully scoop out an indention in the middle of the cake then fill with buttercream. Place the top of the cake back on the filled bottom then generously ice with the remaining buttercream.
Sprinkle the cake with the coconut and gently press into the frosting on the sides until covered. Remove the wax paper strips and serve.