To a large pot, add the 8 cups (2 L) of cold water and sprinkle in the agar agar powder. Whisk to combine. Let the agar dissolve in the water for about 5 minutes, whisking occasionally.
2. Step
Add the sugar and whisk again to combine.
3. Step
Place the pot over medium heat and bring to a simmer. Simmer for 3 minutes, until all of the agar and sugar have dissolved, then remove from the heat. Do not keep over the heat for too long or the agar will denature!
4. Step
Pour the agar mixture into a large glass or ceramic bowl with a smooth bottom.
5. Step
Refrigerate overnight, or until set.
6. Step
Make the brown sugar syrup: In a medium saucepan over medium-high heat, combine the brown sugar and water. Stir to dissolve the sugar. Bring the syrup to a boil and remove the syrup from the heat, transfer to a liquid measuring cup, and refrigerate until cool and thickened, about 1 hour.
7. Step
Invert the raindrop cake onto a large plate or cutting board.
8. Step
Top with the roasted soybean flour and brown sugar syrup.
