Start by making the tangzhong. Place bread flour and milk in a saucepan. Heat over medium-high heat and whisk until it comes together, about 1 minute. Set aside and allow to cool completely.

Combine the yeast and milk and let it rest for 5 minutes, allowing the yeast to become active. Combine the bread flour, cake flour, salt, and sugar in a large bowl of a stand mixer. Add the tangzhong, milk, and egg to the bowl and mix until it just comes together.

While the dough begins to knead, add the butter one tablespoon at a time. Knead the dough for 5 minutes. If the dough is too sticky, add 1 tablespoon of flour at a time. If it is too dry, add one tablespoon of milk at a time.

Transfer to an oiled bowl, cover, and let rest for 1 hour or until it has doubled in size. Once doubled, punch down and knead. Prepare a baking sheet with parchment paper.

Divide the dough into 9 equal portions. Shape into round balls and place on the baking sheet. Cover with plastic wrap and let rest for 30 minutes, or until doubled in size.

Take one of the round portions and roll out to a 4 to 5 inch circle. Make sure the center of the dough is a bit thicker than the edges. Place 65 grams of ogura-an in the center. Wrap the dough around the ogura-an and pinch the edges together.

Once pinched shut, flip the dough (seam side down) and gently rub on a countertop to help seal and smooth out the bottom. Keep an eye on it and pinch any open seams as you do this. Place back on the baking sheet, seam side down. Repeat with the remaining portions. Cover the baking sheet with plastic wrap and let rest for 30 minutes, or until doubled in size.

Preheat an oven to 375°F. In a small bowl, whisk an egg and 2 tablespoons of milk. Remove the plastic wrap from the baking sheet and brush each of the buns with egg wash. Sprinkle a generous portion of black sesame seeds on the center of the bun. Place in the oven and bake for 16 to 19 minutes or until golden brown.