In a large mixing bowl, combine flour, baking powder, salt, and cinnamon. Cut cold butter into small cubes and work it into the flour mixture using your fingers or a pastry cutter until it resembles coarse crumbs.

Gradually add milk to the flour mixture, stirring until a soft dough forms. Do not overmix. Turn dough onto a lightly floured surface and knead gently 3-4 times until smooth.

Roll the dough into a rectangle approximately 12x16 inches and about 1/4 inch thick. If dough is sticky, dust lightly with additional flour.

Mash the guava with a fork until it forms a spreadable paste. If using guava paste, soften it slightly by warming. Spread the guava evenly over the dough, leaving a 1-inch border on all sides.

Starting from the long side, carefully roll the dough into a tight log, sealing the edges by pinching them together. Wrap the roll tightly in cheesecloth, tying the ends securely with kitchen string.

Fill a large pot with water and bring to a boil. Place a steamer basket inside, ensuring the water does not touch the basket. Carefully place the wrapped dough roll in the steamer basket.

Cover the pot and steam for 90 minutes, checking occasionally to ensure the water has not evaporated. Add more boiling water if needed.

While the duff steams, prepare the butter-rum sauce. In a saucepan, melt butter over medium heat. Add sugar and heavy cream, stirring constantly until sugar dissolves and mixture comes to a gentle simmer.

Remove sauce from heat and stir in vanilla extract and a splash of rum to taste. Keep warm until serving.

After 90 minutes, carefully remove the duff from the steamer. Let it rest for 5 minutes, then unwrap the cheesecloth. Slice the duff into 1-inch thick rounds.

Serve warm slices on individual plates, generously drizzled with the butter-rum sauce.