This can be stored in an airtight container for 3 days at room temperature and upto a week in the fridge.
You can replace the rum with 1/3 cup water or 1/3 coffee (1 tsp coffee powder dissolved in 1/3 cup water).
The cake will bubble and rise while in oven and settle down once cooled.
You can bake the same batter in a 6 inch pan at 160 C / 325 F for 45- 50 minutes.