Mix flour, salt, and water until dough comes together. Roll out into a rectangle, rest, then roll out to 15"x15" square. Brush butter/fat mixture on bottom half, fold in half and roll out to an 18"x18" sq. Add butter to the bottom half, fold over and repeat again. Roll the dough into a cylinder and rest in fridge until it is solid. Cut into 1" slices, and fill cupcake tin to make cups.

Heat cream for custard. Add flour to cream little by little. Mix egg yolks and sugar and add with Shoyu, mirin, and seasoning packet to combine.

Fill cups and bake at 500 F for 15-20 mins until browned. Cool for 5 minutes and cool on a wire rack.