In large bowl, combine whole wheat flour, whole wheat + cornstarch, sugar and salt.

Beat egg in small bowl; add to dry ingredients.

Add milk and water (both kept at 86F)--I heated in microwave a briefly let rest

Use fingers (or wooden spoon) to gently mix the ingredients together until combine. Keep mixing until it forms loose, sticky ball, about 2 minutes.

Sprinkle flour on a clean surface and transfer the dough. Knead and punch the dough, about 5 minutes.

Stretch dough 10 inches, then put the butter on top of the dough. Fold and continue kneading until the butter is mixed in thoroughly and the dough is smooth.

Start banging the dough on the counter (or the flat surface you are working on) and turn it to 90 degrees and "punch" it with the heel of your hand. Continue for about 10 minutes, or until the dough is smooth and silky. DO a windowpane test to see if the gluten has developed.

Shape dough into a ball by pulling all sides of the ball to the bottom and pinching them together.

Place the dough in bowl (seam bottom down) and cover with plastic wrap. Let dough rise until it has doubled in size, about 1-2 hours. Once doubled, poke a hole in the center. If it doesn't close, then it's ready.

Press the dough with your hands to release the gas in the dough and deflate it. Fold the side toward the middle. Then fold in thirds. Keep the seam side down. Shape dough into a ball, rotating clockwise with both hands.

Use dough scraper (I used a spatula) to cut dough into 8 equal parts. Shape each piece into a nice round ball, pulling from all sides and tuck at the bottom.

Put dough on baking sheet lined with parchment paper. Cover with plastic wrap to prevent drying. Let rest 15 minutes at room temperature.

After resting, flatten dough with hands, fold in thirds and shape into balls, press down and stretch about 3 inches. Then put a spoonful of anko (red bean paste) in the middle of the dough. Wrap the dough tightly around and return to baking sheet,

Cover with plastic wrap and let rise until dough has doubled, about 30 minutes. Preheat oven to 400F.

In small bowl, add 1 egg and 2 tablespoon water. Whisk well. Use pastry brush to brush all sides of the dough with an egg wash.

Sprinkle black sesame seeds or chia seeds on top.

Bake at 400F for 13-15 minutes. Once baked, transfer to wire rack and let cool. Serve immediately or store in airtight container or bag. Enjoy!