Mix milk, water and dry yeast
Mix flour, sugar, and salt into 1 and refrigerate over night (at least 8 hours)
Roll out the dough into a 12 x 20 rectangle using saran wrap.
Put sweet potato mash in the top half, cover the bottom half on top, and press down lightly.
Cut 4 lengthwise into 6 strips
Lay 5 on top of a baking sheet. Then, brush the top with milk, then sprinkle with black sesame seeds and bake at 180 degrees for 15minutes