3-4 hours before you’d like to prep & bake manapua, prepare the dough by thawing it out. Place the frozen dinner roll dough balls on a parchment-lined baking sheet, arranging them at least 2 inches apart from each other. Lightly spritz a large piece of plastic wrap with nonstick cooking spray. Gently place it over the frozen dough, oil side facing down. Set aside for 3-4 hours, until the dough is completely thawed & looks puffy or swollen.
Once the dough is thawed, begin prepping the char siu filling. Add the grapeseed oil to a large skillet over medium heat. Once the oil is hot & shimmering, add in the yellow onion & char siu pork. Season with a small pinch of kosher salt & 1/2 teaspoon white pepper. Cook, stirring occasionally until the onion is translucent & the pork is rendered slightly. Add the water chestnuts & green onion to the skillet, stirring to combine. Cook 1-2 minutes longer, just until the green onions are softened & fragrant.
Add the dark brown sugar to the skillet, stirring to coat the pork mixture. Cook until dissolved, 1-2 minutes. Meanwhile, whisk together the chicken broth, oyster sauce, hoisin sauce, & sesame oil. Once the brown sugar is dissolved, add the chicken broth mixture to the skillet. Stir to combine. While stirring occasionally, bring the mixture to a simmer & allow everything to bubble together for 1-2 minutes. Stir in the cornstarch slurry. Cook 1-2 minutes longer, until thickened. Carefully transfer the char siu filling to a large bowl or plate to cool before assembling the manapua, or transfer to an airtight container & store in the refrigerator for up to 5 days.
Lightly dust a clean work surface with flour. Flatten one dough ball using the palm of your hand into a circle roughly 3 1/2 inches in diameter. Add a heaping spoonful (roughly 2 tablespoons) of the prepared char siu filling to the center of the dough. Starting on one side of the dough & working your way around the circle, use your fingers to gather & pinch the edges of the dough until the filling is fully enclosed & the bao is sealed. Use your hand to smooth & press the seal flat. Place the assembled bao, seam side down, on the parchment-lined baking sheet. Repeat with remaining dough balls & char siu filling until all 12 manapua are assembled.
Place the lightly oiled plastic wrap over the assembled manapua, oil side facing down. Set aside to proof for 30 minutes, at which point the dough will look puffy & swollen again. This is also a great time to preheat the oven for baking. Preheat the oven to 350 degrees F, ensuring a rack is positioned in the center of the oven.
Lightly brush the tops of the bao with egg wash, then transfer to the oven. Bake the manapua 18-20 minutes, or until beautifully golden brown.
Remove the baked manapua the oven & allow to cool slightly. If desired, brush the tops of the bao with honey wash for a crave-worthy sticky-sweet finish. Serve immediately. Enjoy!