In a small bowl, combine 4 tablespoons of warm water, half the sugar and the yeast. Stir until dissolved and wait about 10 minutes, until it foams. If it does not foam, the yeast is dead or the water was too hot: start again.
In a large bowl, add the flour, salt, the rest of the sugar and the yeast mixture. Start mixing by hand.
Add the margarine, vanilla and the whole egg. Mix well.
On a floured surface, knead the dough. If it sticks to your hands, add a little more flour.
Keep kneading for 8 to 10 minutes. The dough should be elastic and smooth, neither dry nor wet, and spring back slowly when pressed.
Put the dough in a bowl, cover with plastic wrap and let it rise for at least 1 hour, until roughly doubled.
Turn it onto a floured surface and knead briefly. Cover with a towel and rest for another 10 minutes.
Divide the dough into 3 equal pieces and shape each into a thick rope.
Roll each piece into a long rectangle. Put the guava paste or arequipe down the centre and fold the dough over it, rolling it closed.
Join the two ends to form a ring. Repeat with the other pieces.
Place them on a baking tray lined with parchment paper, cover, and let them rise for about 20 minutes.
With scissors, snip cuts around the outer edge of each ring. These open as it bakes and let steam escape.
Preheat the oven to 320 °F (160 °C). Brush the roscones with the beaten egg and the melted butter.
Bake for 15 to 20 minutes, until golden.
Decorate as you like: arequipe spread on top with coconut and sprinkles, or simply sugar and cinnamon.