To prepare the dough, in the bowl of a stand mixer fitted with the paddle attachment, add flour, baking powder, sugar and salt. With the mixer on its lowest speed, mix in butter. Gradually pour in water and continue mixing until the dough comes together.
Dust a clean surface with flour, knead dough and form into a ball. Let rest for 10 minutes.
Roll dough out into a rectangular shape. Using a pastry cutter, divide dough into 3 roulades. Place sliced guavas over each dough, leaving about 1/2-inch border at the ends.
Roll dough into a 2-to 3-inch roulade, fold and seal both ends firmly by pinching together.
Tear a large enough piece of heavy-duty foil. Carefully transfer the roulade to one end of the foil, then roll loosely, allowing a little space for the dough to expand. Secure tightly by folding both ends upwards (this prevents water from entering during the steaming process). Repeat with the remaining dough.
Place the roulades over the rack of a 10-quart (10 L) oval roaster. Add a little water to reach 1 inch just below the rack. Place over high heat and bring to a boil, then cover tightly with a lid. Reduce heat and steam for 30 minutes, while replenishing the water level occasionally.
Meanwhile, to prepare the rum sauce, in a blender, combine 1/2 cup diced guavas, cream, condensed milk, evaporated milk, vanilla, nutmeg, sugar and rum and puree.
To prepare the guava crème, in a bowl, whisk cream to stiff peaks. Fold in diced guavas. Cover with plastic and keep in the refrigerator until ready to use.
Carefully remove the roulade from the pan (as hot steam will escape). Remove and discard the foil. Place guava duffs on a platter and slice them into 2-inch thick pieces on the diagonal. Pour rum sauce over and serve garnished with guava crème and mint leaves.