Make the bread wrap: dissolve the sugar and yeast in warm (not hot) water in a large bowl. Let it stand for five minutes. Then, add the milk, eggs, oil, and salt. Stir gently. Add 1 cup of wheat flour and 2 1/2 cups of white flour and stir to form a soft dough. (Reserve the 1/2 cup of white flour for kneading the dough. It is very sticky).
Sprinkle white flour onto a bread board. Gently turn the dough onto the board and knead for 8 minutes. The dough is very sticky, so use flour as needed to sprinkle on the board, your hands, etc. to keep kneading.
After kneading the dough, spray cooking spray inside a clean bowl. Place the kneaded dough in the bowl and cover with plastic wrap. Place in a warm, draft free area for an hour to rise.
Punch the dough down and let it sit for five minutes after it is finished rising.
Sprinkle flour onto the bread board. Work in small batches. Take some dough and roll it out until thin. Cut into rectangles about 4 x 6 or slightly larger if desired.
Place chilled filling inside the center of each dough rectangle. Then bring the sides of the dough together on a diagonal, pinching until sealed. I use a bit of water to seal the ends.
Line baking sheets with parchment paper. Preheat the oven to 375 degrees F
Place assembled bierocks seam-side down on the parchment covered baking sheet. If they are mishappen, gently pat them into rectangles or squares. Cover with a clean dish towel. Let them rest and rise again for 20 minutes while the oven preheats.
After the final proofing, bake bierocks for 15 minutes. They sound hollow when tapped which indicates they are done!
Allow to cool at room temperature and store uneaten ones in the fridge or freezer. Unused filling can be cooked or heated and added to soup broth for a tasty soup.