In a medium pot over medium heat, combine rock sugar and 200 grams cold water. Bring to a boil and reduce to a simmer. Cook until sugar is melted. Transfer to a medium bowl. Cool completely, at least 2 hours.
Meanwhile, in a large skillet over medium heat, add the black sesame seeds (do not use any oil). Heat for 3 – 5 minutes until fragrant, stirring often. Transfer to a large plate to cool completely, at least 1½ hours.
In a food processor or blender, add the cooled black sesame seeds. Process until fine. Add the flours and starch, do a quick purse. Add the cooled sugar syrup and another 200 grams cold water. Mix until blend.
In a large bowl, place the sieve on top and cover with cheesecloth. Pour the sesame mixture into the cheesecloth. Hold up the cheesecloth carefully and squeeze out as much liquid as possible. Keep the liquid and discard the seed mixture.
In a large wok or pot, put in the steaming rack and add water that reach 1 inch below the top of the rack. Bring the water to boil over medium-high heat.
In the meantime, lightly brush the inside of the sheet pan with vegetable oil. Use paper towel to remove extra oil if needed. Pour about 75 milliliter of sesame liquid. The liquid should fully cover the bottom of the pan when the pan is level. If the liquid can't cover the pan, add a little bit more liquid.
Transfer the sheet pan with the liquid to the wok and on top of the steaming rack. Carefully adjust the rack to ensure the sheet pan is level again. Cover with a lid and steam for 3 – 4 minutes. Remove from heat and cool for about 5 minutes.
Lightly brush a thin layer of vegetable oil on top of the sesame sheet. Using a thin spatula, lift up a corner and pull the whole sheet out gently, and lay it back on the pan. Roll the sheet from the short length side to create a roll. Transfer to a medium plate to cool.
Rinse and wash the sheet pan. Dry with paper towels. Repeat the steps to make two more rolls. Rest the rolls at room temperature for 30 minutes to cool. Wrap the whole plate with plastic wrap. Chill in the fridge for up to 6 hours.
When ready to serve, lightly grease a sharp knife with vegetable oil. Cut each long roll into 3 parts. Transfer to a serving plate. Sprinkle with white sesame seeds and serve.