In a medium bowl mix together the pineapple juice, yeast and granulated sugar. Then let it sit for 20 min. for the yeast to activates.
In another bowl mix the buttermilk, eggs, vanilla, and butter together, and set aside.
In a different big bowl, sift together the flour, milk powder, brown sugar and salt. Now stir in the yeast and buttermilk mixtures and mix using a wooden spoon until you get a well combined dough.
Place the dough on a working surface and knead for 15 min. until it becomes soft and elastic (this whole process can also be made using a standing mixer with the dough hook attchment).
Place the dough in a bowl previously oiled and cover it with a kitchen towel. Let the dough rest for two hours or until it doubles its size.
Once this happened, place the dough on the working surface again and cut it into 12 same-size portions.
Form balls with each portion to create buns and place them, without too much space between each other, in a previously greased rectangular baking pan.
Cover the pan with a kitchen towel and let the buns rest again until they double their size (about an hour).
20 minutes before the buns have doubled their size, preheat the oven at 375° F (180ºC)
For the egg wash, in a small bowl just mix the egg yolk and milk.
Once the buns have doubled their size, brush them with the egg wash and bake at 375°F (180ºC) for 20 minutes or until they turn golden brown.
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