Heat up water in microwave for 40 seconds. Combine with cold milk in a bowl or large cup.
Add sugar, yeast, and oil to milk/water. Let sit for 5-10 mins until a yeast froth forms.
In the meantime, combine grounded black sesame seeds, all-purpose flour, whole wheat pastry flour, and baking powder in a large mixing bowl.
If using a stand mixer: use the dough hook attachment and set mixer to #2 speed. Add the yeast/milk slowly to the dry ingredients. Let the mixer run for 8 minutes. If the dough is too dry, add a tsp of milk (does not need to be warm) at a time until dough forms into a smooth ball.
If kneading by hand: after combining all the dry ingredients thoroughly, make a dent in the middle of flours and pour the yeast mixture. Combine with hands and knead for 8 minutes forming a smooth ball. If dough is too dry, add 1 tsp of milk (does not need to be warm) at a time till a smooth ball forms.
Put a plastic wrap, warm wet dish towel or a large pot lid over the bowl and let the dough rise somewhere warm for 1-2 hours or until the dough doubles in size. Can also be left in the fridge covered overnight to be steamed the next day.
After dough has risen, put on a flat work surface and rollout dough into a 8x10 rectangle. Fold the top and bottom of dough inwards. Then fold the sides of dough inwards to make a neat package. Roll the dough again into an 8x10 rectangle.
Roll dough up long ways (like a Swiss roll cake) and cut dough into 5 equal pieces. Cut up 5 pieces of parchment paper and place mantous on top.
Fill a steamer with COLD water and put mantous inside, put a lid on top. Set stove to medium and when the water comes to a boil, set your timer to steam the mantous for 15 minutes. After mantous are cooked, turn off stove but don't open the lid for a couple of minutes. Enjoy!