Sift the flour into a bowl. Stir in the salt and the sugar.
Blend the butter and shortening into the flour mixture with a pastry cutter or with two knives, until fairly well blended.
Whisk the egg yolks with 3/4 cups water. Stir in the 1/2 cup of water/egg mixture, a little at a time until the dough starts to come together smoothly. Keep kneading the dough, adding more water/egg a little bit at a time as necessary (you made need a few tablespoons extra of water), until the dough is smooth. The dough will seem a bit shaggy until it has thoroughly chilled.
Cover the dough with saran wrap and refrigerate for about an hour. (Dough can also be kept overnight (or a couple of days) in the refrigerator) Dough should be soft and smooth, and not elastic - if you poke a hole in it with your finger, the indentation should remain.
Prepare empanada dough and chill.
Poach the chicken: Place the chicken breasts in a pot with the bouillon and bay leaf, and cover with water or chicken broth.
Bring to a boil, and simmer over low heat for 15 to 20 minutes, until chicken is cooked through. Let cool in the broth.
Shred chicken into small pieces, and moisten with 1 or 2 tablespoons of the broth. Set aside.
Heat oil in a skillet. Add chopped onions, paprika, cumin, chile powder, sugar, and salt and pepper to taste.
Cook over low heat for 15 to 20 minutes, until onions are soft and clear, and mixture is golden brown.
Remove from heat and stir in the chicken. For best flavor, refrigerate filling until the next day.
Stir chopped hard boiled egg and olives into filling mixture.
Separate empanada dough into golf ball size pieces, and roll each one into a smooth ball. Let rest 5 minutes.
Preheat oven to 220°C.
On a floured surface, roll each ball of dough into a circle about 6 inches in diameter.
Spoon 2-3 tablespoons of filling in the middle of the circle. Brush the edges of the dough along the bottom half of the circle lightly with water.
Fold the top half of the circle of dough over the filling to form a semicircle, and press edges together firmly to seal.
Brush the pressed edge with a little bit of water, and fold the edge over itself, pinching and crimping as you go to make a braid-like effect. Place empanadas on a baking sheet.
Mix egg yolk with water and brush mixture lightly over entire surface of each empanada.
Bake for 15 to 20 minutes, until golden brown and slightly puffed.