Preheat the oven to 425 degrees. Prick the sweet potato all over with a fork and place it on the top rack of the oven. Place a baking sheet on the lower rack underneath to catch any potato juice. Roast for 35-40 minutes
Remove the potato and let it cool for 5 mintues. Pull the skin off the potato and add the flesh to a mixer.
Gently melt the chocolate in a doulble boiler on the stove over hot water and pour the chocolate into the mixer with the potato. Whip for about 3 minutes until everything is light and fluffy.
Spread the icing evenly over your brownie of cake and pop it into the fridge for 15 minutes to set.
This icing will keep well in the fridge in an air-tight glass container for 5 days.