Add a little booze to upgrade the typical coffee-flavoured buttercream.
In the bowl of a mixer, beat the butter until smooth. Add the cocoa powder and continue to beat until incorporated. Beat in 1 cup of the icing sugar until the icing is fluffy.
Dissolve the instant coffee into the the spice rum, and beat into the icing with the mixer on medium speed. Taste the icing and beat in the remaining icing sugar and cocoa powder to taste.