In a small bowl, combine 4 tablespoons of warm water, half of the sugar, and the yeast. Stir until dissolved. Wait about 10 minutes until the yeast starts to rise.
In a large bowl, add the flour, salt, the rest of the sugar, and the previous mixture. Start mixing by hand.
Add margarine, vanilla, and the whole egg. Mix well.
On a floured surface, knead the dough. If it sticks to your hands, add more flour.
Keep kneading for about 8 to 10 minutes. The dough should be elastic, neither too dry nor too wet.
Transfer the dough to a bowl and cover with plastic wrap. Let it rise for at least 1 hour.
After this time, place the dough on a floured surface and knead a bit more. Cover the dough with a towel and let it rest for another 10 minutes.
Next, divide the dough into 3 equal parts. Shape each piece into a thick rope.
Using a rolling pin, roll out each dough piece into a long rectangle. Place the guava paste or dulce de leche in the center and fold the dough over (Roll the dough covering its center).
Join the two ends together forming a loop. Repeat this process with the other two pieces of dough.
Place the pastries on a baking tray lined with parchment paper, cover, and let them rise for about 20 minutes.
With a pair of scissors, make cuts in the dough around the outer edge.
Preheat the oven to 160°C (320°F). Brush the pastries with the beaten egg and melted butter.
Bake for 15 to 20 minutes, or until golden brown.
Decorate to your liking, spreading dulce de leche on top, then sprinkle with coconut and sprinkles or with sugar and cinnamon.