Step 1: Make the Yudane (Optional, skip this step if making straight method)

1. In a small pot, heat 60 ml of water until it reaches a boil.
2. Add the 40 g of flour, stirring quickly to form a thick, paste-like mixture.
3. Remove from heat, let it cool to room temperature, and store in the refrigerator overnight or for at least 4 hours. This mixture helps create soft and fluffy bread.

Step 2: Prepare the Dough

1. Combine wet ingredients: In a large bowl, whisk together the whole milk, egg yolk, sugar and yeast. If skipping the yudane, add in the water as well. 
2. Add dry ingredients: Add the flour, salt and yudane (if skipping yudane, add the additional flour). Mix well until a shaggy dough forms.
3. Knead: Add the softened butter and knead the dough on a clean surface for about 10-15 minutes until it becomes smooth and elastic. You can also use a stand mixer with a dough hook attachment or bread machine.

Step 3: First Proofing

1. Place the kneaded dough in a lightly oiled bowl, cover it with a damp cloth or plastic wrap, and let it rise in a warm place (about 25-28°C) for 1-1.5 hours or until it doubles in size.

Step 4: Shape the Anpan

1. Punch and divide: Once the dough has doubled, gently punch it down to release air. Divide it into 8 equal pieces (approximately 50 g each). You may also divide it into to 6 pieces for larger buns.
2. Rest: Roll each piece into a smooth ball and let them rest on the bench, covered with a cloth, for 15 minutes. This relaxes the gluten, making it easier to shape.
3. Fill: Flatten each dough ball into a circle, about 8 cm in diameter. Place 37 g g of red bean paste (anko) in the center.
4. Seal: Pull the opposite edges of the dough up and over the anko, pinching them together to seal. Then pinch together the opposite corners together. Roll the filled dough ball gently in your hands to create a smooth surface.

Step 5: Second Proofing

1. Place the filled dough balls seam-side down on a baking sheet lined with parchment paper.
2. Cover loosely and let them proof for 30-45 minutes, or until they become puffy and almost double in size.

Step 6: Prepare for Baking

1. Preheat your oven to 200°C (390°F) and remove cover from the buns to let them dry out a little.
2. Egg wash: Whisk together the egg and water. Brush the tops with egg wash. This gives the buns a beautiful golden-brown color.
3. Add topping: Put the black sesame seeds in a small bowl. Dip any utentil with a flat round bottom (I use a wooden rolling pin) and dip it into the egg water. Then dip into the black sesame seeds and stamp it onto the buns. Alternatively, you can just sprinkle the sesame seeds or poppy seeds on top. 

Step 7: Bake

1. Bake in the preheated oven at 200°C for 10-12 minutes. If making 6 buns, bake for 12-15 minutes.
2. Keep an eye on the buns to avoid over-browning. They should be golden and sound hollow when tapped.

Step 8: Cool and Serve

1. Let the buns cool on a wire rack.
2. Enjoy your freshly baked anpan warm or at room temperature!