- Heat the oven at 350F. Line a 9-inch square baking pan with foil and coat it with the help of a cooking spray. Take a pan; mix brown sugar, rum, pineapple and cinnamon together. Take a separate medium bowl; beat baking soda, salt, baking powder and flour well.
- Add sugar and butter in a large bowl and beat with the help of a mixer for 5 minutes. Add egg and vanilla and beat again. Now add flour mixture and buttermilk alternately to sugar mixture, start and end with flour mixture and whisk. Toss cranberries and pour pineapple mixture over.
- Bake for 35 to 40 minutes or until lightly brown. Place on the wire rack for 5 minutes.
- Transfer to the serving plate and enjoy Cranberry Coffeecake at once.