Chop or shred pot roast; place in large microwave-safe dish. Cover, vent and microwave until heated through, stirring occasionally. Stir in spinach and hoisin sauce. Microwave until sauce is thickened and beef is coated with sauce. Set aside.
Cut parchment paper to line basket of stove-top steamer, cutting hole in center as needed. Add water to steamer, making sure water level is below basket. Bring water to a boil.
Meanwhile, place biscuit dough pieces on cutting board. Flatten each piece of dough into 3-inch square, extending corners; press corners onto board with thumbs. Place about 1 tablespoon of beef mixture in center of dough square. Bring two opposite corners up and over filling and pinch together; bring remaining two corners up and over filling, pinching all seams and corners together to form square bun. Repeat with remaining dough and filling. Turn buns over until ready to cook. Place buns into steam basket in batches, keeping at least 2 inches apart. Cover and steam 7 minutes or until dough reaches temperature of 190°F and the filling is 165°F. Gently remove buns from basket with long handled tongs; cool. Repeat until all buns are steamed. Serve bao buns with sauce as desired.