In a spice grinder or pestle and mortar, finely crush the annatto seeds, oregano, cumin seeds and peppercorns. Transfer to a small bowl, add allspice, salt, water, garlic and vinegar and mix well.
Place chicken breasts in a food processor and pulse until broken up, but do not allow to blend to a paste. Stir achiote paste and cabbage into chicken mixture.
Lay out 6 wonton wrappers at a time and in the centre of each one, place 1 heaped teaspoon of mixture. Dip your finger into water and run it around the edge of the wrapper, then fold in half and seal edges with fingertips to make a tostada.
Place on a lightly oiled tray and cover with cling film until ready to cook.
In a deep frying pan, heat the vegetable oil over moderate heat then carefully add tostadas to the pan, cooking 2-3 minutes each side. Remove with a slotted spoon and drain on paper towel.
Combine ingredients for mango sauce and season to taste and serve with the warm tostados..
There is a little work in making these, but the appreciation of your guests will make it all worthwhile.