In a large mixing bowl, combine the flour, baking powder, and salt.
Cut in the cold, cubed butter using a pastry cutter or your fingers until the mixture resembles coarse crumbs.
In a separate bowl, whisk together the milk, sugar, vanilla extract, and egg. Gradually add this mixture to the flour mixture, stirring until a dough forms.
Turn the dough out onto a lightly floured surface and knead briefly until smooth. Cover with plastic wrap and let it rest for 30 minutes.

In a small bowl, combine the guava paste or guava jam, brown sugar, and ground cinnamon. Mix well.

Roll out the rested dough on a lightly floured surface into a large rectangle, about 1/4 inch thick.
Spread the guava filling evenly over the dough.
Starting from one long side, roll the dough up tightly into a log.
Wrap the log in a clean kitchen towel or cheesecloth and tie the ends securely.

Fill a large pot with enough water to come halfway up the sides of the wrapped duff. Bring the water to a simmer.
Place the wrapped duff in a steamer basket or on a rack set above the simmering water. Cover the pot and steam for about 1.5 to 2 hours, or until the dough is cooked through and a toothpick inserted in the center comes out clean.

In a medium saucepan, melt the butter over medium heat.
Stir in the granulated sugar, evaporated milk, and vanilla extract. Cook, stirring constantly, until the sugar is dissolved and the sauce is smooth.
Stir in the guava puree or guava jam and cook for another 2-3 minutes until heated through.

Remove the duff from the pot and unwrap it from the towel or cheesecloth. Let it cool slightly before slicing.
Serve the guava duff warm, drizzled with the guava sauce.
"Enjoy your meal!"