Mix the cocoa with the hot water until you get a smooth paste.
Melt the butter with the sugar in a saucepan over low heat.
In a bowl, mix the butter mixture for about 5 minutes at medium speed.
Add the egg, vanilla and cocoa paste.
Set the mixer speed to low.
Add the flour and mix until just added.
Finally add the sour cream.
Divide the batter over the cupcake tins.
Bake in the oven for 20 minutes, let stand for 5 minutes, and then remove them from the baking tin.
Allow to cool further to room temperature.
Put the cream in a bowl and mix with the mixer until it is stiff.
Optional: if you want a colored frosting add the desired color to this step. Make it a little more intense than you want, because you will mix the cream with mascarpone and that will soften the color a bit.
In a second bowl, mix the mascarpone with the powdered sugar until well blended.
Add the whipped cream to the mascarpone mixture and mix with a spatula until done.
Put the frosting in a piping bag and pipe a tuft of frosting on each cupcake. I used Wilton tip 1A for this.
Put some cotton candy on the frosting and garnish with a spider (do warn everyone that the spider is not edible).
Optional: sprinkle some colored sugar on the cotton candy