Place the Shiratamako in a large microwave-safe bowl.
Add 1/4 cup (60 milliliters) of the hot water and mix well with a whisk or spatula until there are no lumps remaining.
Mix in the Joshinko, sugar, and cocoa powder, followed by another 1/4 cup (60 milliliters) hot water until evenly blended. If still too powdery, slowly add another small splash of water.
Cover the bowl lightly with a sheet of plastic wrap.
Microwave the bowl for 1 minute.
Fold the dough with a lightly wet spatula to evenly mix, then microwave again for 1 minute.
Repeat this process and microwave for 30 more seconds, then, if needed, another 30 seconds to create an evenly glossy and sticky dough.
Dust a large work surface with some of the Kinako cocoa mixture.
Place the hot, just microwaved dough on the kinako-covered surface and dust the top with a little more of the kinako mixture.
Quickly roll the dough with a rolling pin, dusting as needed, into a thin 9 inch (23 centimeter) square sheet about 1/16 inch (1.5 millimeters). If necessary, use your hands to lightly stretch the dough into a more even shape.
Use a sharp knife or pastry wheel to cut the sheet into 9 even squares (about 3 x 3 inches/7-8 x 7-8 centimeters).
If you did not get the squares completely even, use a sharp knife to cut off any excess pieces.
Place a 1/2 ounce (15 gram) spoonful of either the chocolate bean paste or chocolate ganache in the center of a square.
Fold the square over from corner to opposite corner to form a triangle. If needed, wet the edges lightly with water to help them stick.
Repeat the process with the remaining squares and filling.
Brush off any excess Kinako.
Serve immediately or store in an airtight container at room temperature for up to a day. Do not refrigerate.