In a large bowl mix together warm water, instant yeast, oil and sugar. Stir in 2 cups pastry flour, 2 cups flour and salt. Transfer to a floured surface and knead, adding remaining flour a little at a time, until the dough is smooth and elastic, about 8-10 minutes. Place into a greased bowl, cover and let proof for 1 hour or till double in bulk.
In a large skillet add oil, sauté the onions and add the pork and sausage. The meat is already cooked, so you don't have to cook it for very long, about 3 minutes. Add in the wine, soya sauce and the hoisin sauce. Cook for few minutes. Pour into a bowl and allow to cool. After dough has risen, transfer to a floured surface (try not to deflate the dough to much). Roll dough into a long rope shape. Using a sharp knife cut dough into 20-24 pieces. Roll each piece into a ball. Flatten each piece one at a time in your palm. Place a heaping tablespoon into the center of the dough. Pinch the dough together with your fingers. Then shape into a ball. Place onto a parchment lined cookie sheet. Seam side down about two inches apart. Cover with plastic wrap for 45 minutes. Glaze each bun with the egg yolks and milk allow to sit for 5 minutes then repeat again. Bake in a preheated 350F oven for about 20 minutes, or until the tops are browned lightly. Cool on a wire rack.
*NOTE - you can buy cooked BBQ Pork at any Chinese market or restaurant.