You really can't go wrong with fudgy chocolate icing on cakes or brownies. Even better if it's homemade. Powdered sugar is normally one of the key ingredients to making frosting, but if you don't have any on hand, try this boiled recipe with granulated sugar. This makes enough for a two-layer cake and is actually cheaper than store-bought frosting in the long run.
Add sugar, cocoa and milk into a medium-sized pot and mix well.
Put the pot on the stove and cook on high until it begins to boil, stirring constantly to avoid scalding or burning. Once mixture starts to boil, remove from the heat.
While mixture is still hot, add butter and vanilla. Mix well to fully incorporate.
If you like wet icing, pour the warm icing on top of the cake. If you want the icing to seep into the cake, poke some holes into the cake using a wooden dowel. The cake will look messy, but will be very chocolaty.
Otherwise, let the icing cool for about 20 minutes. Then beat the icing for about 5 to 10 minutes until it's thick enough to spread on the cake.