Combine the yeast and warm water in a small bowl. Let mixture sit until foamy, about 5 minutes. In a large mixing bowl, combine eggs, milk, butter, sugar and salt.
Add the yeast mixture and 3 cups of flour. Stir with a wooden spoon until the powders are moistened and the ingredients are incorporated. You will have a sticky mixture at this point.
Gradually add remaining flour a few tablespoons at a time to help with the stickiness and just until a soft dough gathers in the center of the bowl. You won't need the entire amount of the remaining flour.
Turn the dough over on a floured board. Knead the dough until it is smooth and elastic. Dust your hands and flour minimally as needed to help with the stickiness as you knead. Shape the dough into a ball, place it in a bowl and cover with a clean towel or plastic wrap. Let it rise for 1 and 1/2 hour. At this point, you can make the topping dough.
TOPPING DOUGH: Combine milk powder, flour, salt, sugar, baking powder and baking soda. Break any lumps in the powder with your fingers. Add the margarine, egg yolk and milk. Mix everything with your hands until the mixture gathers into a dough. Shape it into a log and wrap it in plastic. Set aside at room temperature.
After the rise, gently punch the bread dough down and divide into 10 portions. Shape each portion into a ball and arrange them on a greased baking sheet with about 2 inches of spaces apart. Cover the buns loosely with a plastic wrap and let them rise for 1 hour, or until the sizes are doubled.
Preheat oven to 350 F. Using a knife, Slice the topping to yield 10 portions. Shape each into a ball and flatten into a disc that is NOT larger than the buns. Place each topping over the buns using a metal spatula, making sure that the buns are not entirely covered. The topping will expand upon baking so it needs to be smaller than the bun. Lightly score some lines across the topping to resemble a pineapple.
Brush the topping with egg yolk for a nice, golden shine. Bake the buns for 15-20 minutes or until the tops are cracked and rough, and the color is lightly golden.
Topping dough adapted from The Woks of Life