Prepare the Tangzhong Starter: Whisk together bread flour and water in a bowl until well blended and lump-free.

Cook the Tangzhong mixture over medium heat, stirring constantly, until it reaches 65C/149°F (about 3-5 minutes) and has a glue-like texture.

Let the Tangzhong starter cool.

Prepare the Ubi (Purple Yam) Dough: In a mixing bowl, combine bread flour, powdered purple yams, sugar, salt, and instant dry yeast.

Add the cooled Tangzhong starter, warm milk, and eggs to the Ubi dough ingredients.

Mix with a dough hook at slow speed for 1 minute, then switch to medium speed and continue mixing until a dough forms.

Gradually add butter to the Ubi dough and mix for approximately 5 minutes on medium speed until gluten is fully developed (elastic, smooth, non-sticky).

Prepare the Light Dough: In a mixing bowl, combine bread flour, sugar, salt, and instant dry yeast.

Add the cooled Tangzhong starter, warm milk, and eggs to the Light dough ingredients.

Mix with a dough hook at slow speed for 1 minute, then switch to medium speed and continue mixing until a dough forms.

Gradually add butter to the Light dough and mix for approximately 5 minutes on medium speed until gluten is fully developed (elastic, smooth, non-sticky).

Place both the Ubi dough and Light dough in separate lightly greased plastic bags.

Let both doughs rise for about 45 minutes in a warm area until doubled in bulk.

Punch down both doughs and allow them to rest on a lightly floured work surface, covered, for 10 minutes.

Roll out each dough to a rectangle, about 22x10cm.

Place the purple yam dough atop the light dough.

Roll up tightly, beginning at the short side.

Place the rolled dough in a greased 26cm loaf pan.

Cover and let rise until doubled in bulk, about 45 to 60 minutes.

Bake at 170C/340F for 30-35 minutes, or until done.

Remove from pan and let cool on a wire rack.