Mix happy mug cake according to package instructions except use the pineapple juice, rum and coconut milk instead of 3 tablespoons of water.
Transfer the cake batter mixture into your favorite summer ceramic cup and microwave it for 1minute and 30 seconds. Remove cup from the microwave, be careful cup will be hot. Cool and set aside.
Pipe or dollop whipped cream on top of cake then sprinkle some grated coconut (optional).
Place the skewered pineapple slice and cherry on the topside of the cake or use an umbrella cocktail stick for a summer presentation look.